I have never found a Blood Bowl conversion. Primarily, this stems from a couple of reasons as far as I can tell: 1) the teams in context of the WFRP world make no sense and 2) the game too closely resembles the NFL, i.e., American Football.
That said there is the game of Snotball as described in WFRP 1e (first edition) Middenheim: City of the White Wolf. On p. 90, the game is described as follows:
Snotling Football
Snotball (Middenheim Rules Version) is a City craze and some people come to the City just to follow this ancient sport. Unfortunately, Snotball matches do attract many Football Hooligans and violent confrontations between fans of rival teams are not uncommon.
Already you see the resemblance to football as the British know it... "Go Man U!" (Mind you I don't really have an affiliation in any league and only barely follow the Premier League because of my employer.)
In brief, the game is played between two teams of 11 players, on a pitch rougly 200 feet long by 85 feet wide. There is a set of goals at each end of the pitch, both 11 feet wide and 7 feet high.
It's "soccer"!
The snotlings used are fairly young, and must measure between 18 inches and 21 inches so they aren't too heavy and can be booted about easily.
The Snotling is anaesthetized with a curare-like paralyzant; not for its benefit, but because otherwise it might be able to bite and scratch if the leather straps worked loose. Snotball leather-workers are very proud of the craftsmanship, however, and this rarely happens.
Ha! And humane!
Players wear extremely thick, brightly coloured leather jerkins, tousers, shin padding, and a mail coif.
Equipment for a player! A starting kit for a career.
The game is played over two halves of 30 minutes each, with a 15 minute interval at half-time. Two additional periods, of 10 minutes each, are played if the scores are level at full time. Two substitutes per team are permitted.
The aim of the game is to get a Snotling (which is contorted by very tough leather thongs into a roughly spherical shape) into the opposition's goal by any means the players can come up with (kicking, throwing, punching, nutting). Just about the only rule is that no part of the attacker's anatomy my be in contact with the Snotling as it crosses the goal-line. So no rugby-style scrum diving is allowed in scoring a goal (although it can be used to get the Snotling very close to the goal).
Some basic game structure... and that's it.
My initial recommendation for a career: re-skin the Pit Fighter and Judicial Champion for Basic and Advanced career paths, add fluff. I might limit actions to move and feints with the grapple rules and Strength Bonus for kicking "movement" using the SB as the Move attribute calculation.
Update
Marc Torley on G+ suggests another perfect template for a Snotling Football player
BB is a way to follow a team in a sporting event. Snotball is more an excuse to beat up your neighbours. The best template for a snotball player? Thug. Add Flee! And it's ideal!
Have to agree, then you get the career path of Thug.
Fleeing an encounter is difficult in Dungeons and Dragons, Third Edition
Tactically, a creature wanting to end an encounter moves away, usually by taking the action run (Player's Handbook (2000) 127) or double move (PH 126–7). Running is straight-line-only movement of typically 4× the creature's speed but causes the creature to lose its Dexterity bonus to Armor Class and does nothing to obviate any attacks of opportunity that the creature may incur due to its movement. A double move, on the other hand, is safer but slower, allowing the creature to move up to twice its speed and the creature doesn't provoke attacks of opportunity for leaving the space it started from. (Also see Movement on PH 117).1
Likewise, tactically, those who wish to pursue do so by going after the creature, also usually taking the action run or a double move or, if within striking distance, by taking the action charge (PH 124), ending the charge with a grapple attempt (PH 137). Success on the grapple attempt means grappling ensues, and the formerly-fleeing creature's progress is halted unless it escapes the grapple.
This all occurs on the grid or battlemat or whatever playing surface you're using, by the way. Make some room.
Strategically, the Dungeon Master's Guide (2000) provides frankly pretty terrible (albeit nominally realistic and certainly playable) rules for Evasion and Pursuit (70). To summarize, the faster dude wins, yet if speeds are equal make opposed Dexterity checks, with the winner succeeding either in catching up or losing his pursuers, as desired.2
1 This free-space-during-a-double-move idea would later be codified by the 3.5 revision into the action withdraw.
2 If you want more interesting narrative chases, the sort-of D&D 3E compatible Pathfinder Role-playing Game makes its chase procedure available here.
Best Answer
A lot of wizards are inherently imbalanced in WFRP from a combat perspective. This is supposed to be counterbalanced by Tzeentch's curse, but in my experience, that seems to trigger less often than it should. Even an Apprentice wizard can hit harder and more accurrately than a fighter of equal experience, thanks to their magic missle attacks, and it only gets worse when they become Journeymen. I am familiar with this problem, though my party usually doesn't focus on tactical combat. I know of no "simple" Slaughter-Margin like solutions, because the imbalance in combat capabilities is big and I presume you still want the other characters to stay relevant. My solutions, some aspects tried and some theoretical:
Smart enemies
Wizards can cause damage easily, but the enemies should know that, or find it out early in the fight. So they concentrate on the wizards. After the first or second crossbow bolt most wizards are barely standing on 1-2 Wounds, which is a very good incentive for the rest of the party to defend them, and for them to frantically try to stay alive. A few goblins with bows pose minimal threat to any fighter in medium armor, but they can knock a wizard down a few pegs in a single sneak attack salvo. So enemies being reasonable and not stupid, and trying to take out major threats can do wonders for combat encounters. I used this several times with a Journeyman Light wizard and his insane Eye-laser.
Not-smart allies
Wizards are supposed to be feared and hated in most of the Old World, so play with that. Have allies or bystanders around, or simply set up circumstances where blatant and flashy magic use would cause problems. This incentivizes planning on part of the players and makes them think twice about using magic to solve all problems. Because if you are allied with Sigmarite zealots while fighting orcs, subtle magics might remain unnoticed, but a burning missle of death would cause questions, which could lead to burning death on part of the caster. I also did tries out this one, the end result being the party slaughtering both the attacking goblins and the caravan they wanted to save. It was pretty interesting combat- and roleplayingwise as well.
Hordes of mooks
Most wizards don't get real AOE attacks until later careers, and while mooks pose only moderate threat to a well-equipped character, most wizards would have trouble with a lot of zombies/skeletons coming their way. A single one, or even half a dozen pose no real threat to a wizard, but a dozen or two would pose enough of a threat. The important part is that they are unable to blast enough of them in time, so they need tactics and strategy to win. Hordes of mooks are one of the things that armored fighters with Parry and Dodge can handle much easier than wizards, at least in my experience.
A boss with adds
A wizard might be able to cause a great deal of damage, but anything that can take it due to armor and high Toughness (Rat-ogre, Minotaur, etc) can stall them for several rounds. If said boss has minions of their own, especially ones who are smart, combat gets interesting again, as the wizards have to choose between blasting and trying to survive, while the others have to balance themselves between defending the wizards and killing things. I sort of tried this, though the wizard opted to go full blasting and ended up burning a Fate point.
What not to try
Wizards vs wizards. Because rocket tag is not fun or tactical, and the winner usually comes down to luck. It is very much in-line with WFRPs spirit, but it doesn't make for interesting or tactical combat.