steam
Lets say I got a game in my Steam library but I don't have it installed. Can I still buy DLC for it, though?
For example, I have Empire: Total War, but I don't have it installed. Now, I'd like to buy a unit pack since it's on sale as part of Steam Summer Sales. Considering I don't actually have the base game installed, would it work?
It works one way, but not the other. You can use the keys gained in Steam (Right click on game in left-hand menu, View Game CD Key) and activate them for your EA Games account by registering them at https://activate.ea.com, which makes them then appear in your Origin games list for installation. (This applies to most - but not all - EA Games that are available on Origin)
You cannot use Origin codes to apply games to Steam, and Steam won't let you buy DLC for games you don't own though Steam.
[2015 update: You now activate them on Origin though the client. Origin menu, Redeem Product Code.]
As of the Steam Discovery Update on Sept. 23rd, 2014, Steam itself now shows you the DLC you own in the Store page without having to use browser extensions or look at the game properties in your library.
The blue markings on the left side of each DLC entry indicate that I own it already and if I hover over it, there will be a popout that shows that it's in my library.
Note that isn't perfect. In this particular example, Steam shows that I don't own Wonders of the Ancient World, even though I do, but I'm guessing that's a problem with that particular piece of DLC, as it's done that since release. However, Steam still won't let you buy anything you already own, so you're safe from double purchases.
Best Answer
Yes, you can buy the DLC on the Steam store. All you need is a paid copy of the game in your library. You don't need to have it installed to buy it.
The message reads as follows:
This is meant to inform you that you have no use for the DLC - and indeed, cannot buy it - if you do not own the base game on Steam. This is to prevent you from snatching timed free DLC, for instance - and to prevent complaints, obviously.
The DLC will also be automatically installed alongside the game when you download it via Steam - excluding some instances where the DLC actually includes a standalone executable.
If you attempt to buy DLC for a game you don't own on Steam, Steam will tell you - after choosing "purchase for myself" - the following: