BioWare has stated previously that you can gain "effective military strength" through any combination of single player or multiplayer. Single player play contributes "War Assets" while multiplayer contributes "Readiness Rating." These two things multiplied together are a big factor in your ending.
[...] SP can still be played all by itself without any kind of multiplayer or third party and you can still get the absolute best ending. [Multiplayer] just gives you new options to be able to get that ending.
Also in a different interview with Penny Arcade:
You can get best possible outcome in the game by just playing single player if you want, as long as you are willing to do a lot of the side missions and content. However, maybe you only want to do a critical path run of the single player. Well, by playing multiplayer you also increase your galactic readiness rating can affect the final outcome. Using the iPhone Datapad app and the iPad Infiltrator game will also tie into Galaxy at War and update this final rating.
However, it might be challenging to just play the single player. From this PC gamer review:
The game’s still story-driven, and it doesn’t end until you’ve completed the main series of missions. But when you do, what happens in the final cut-scene depends on how many War Assets you have accumulated.
It is possible to get the best ending in single player without playing multiplayer, but it’s twice as hard. All your War Assets only count for 50% of their potential value.
Their intent is to allow people to rush through the single player campaign (if they don't care for the story or side missions) and play a bunch of the multiplayer, and still get the "best ending."
Now that I've finished the game, there's a couple of things I'm going to add to what BioWare has said. While BioWare claims that you can get the "best ending" without playing multiplayer (or the tie-in titles for other platforms), I find this statement to be misleading at best.
It's incredibly unlikely that you will be able to get enough war assets for the "highest tier" endings to Mass Effect 3 without anything increasing your Readiness Rating. You'd have to have played ME1 and ME2 pretty thoroughly, making all the "correct" decisions in your save to import into ME3. Alternatively, you'd have to start from a "perfect" ME2 save import in order to get anywhere close to the number of War Assets you'd need. If you're not importing a previous save, your chances at playing only the single player and seeing the "best" ending are abysmal.
Additionally, you'd have to play this game through very carefully, making sure to maximize your War Assets. Most decisions in the game effect your war assets, and if you don't know ahead of time what the "best" decisions are, you're likely to end the game with significantly fewer assets than you need for some of the "best" endings.
I played fairly well through both previous games, skipped the DLC, and got nearly every war asset possible in ME3. I still fell a good 20-25% short on War Assets required for the "highest tier" ending.
Having said this, the endings are extremely similar. Regardless of how you play, you're highly likely to have very similar options in your ending choices, and each of the endings plays out extraordinarily similarly.
There are three things that affect your multiplayer score. Score is the same as experience in ME3, so the more you score, the faster you level.
Individual Contribution
These only show up on your screen as you earn them. Everytime you get a kill, assist, or help with an objective, a little number will pop up. (I.e., Kill +225). This one is easy to increase - just play the game, and shoot at stuff. Kill / Assist point values are based on how much damage you did to the target. You can get a "Kill" for only 80 points if you knock out the last hitpoint, but hadn't otherwise damage the target.
Individual Achievements
These show up on the end of match results screen, and are worth varying amounts of score. Every medal has differing values of Bronze, Silver and Gold, and the point values scale linearly with weapon kill medals, and geometrically with everything else. This means that getting 25 kills with a pistol and 50 kills with an assault rifle nets the exact same point bonus as getting 75 kills with an assault rifle, but getting 10 headshots and 1 grab is worth less than getting 20 headshots.
Team Achievements
Some medals are awarded team wide. Mission completion and extraction bonuses, for instance, as well as going a certain number of waves without losing a squadmate. Like the individual achievements, these come in Bronze, Silver, and Gold.
To get your score, the game simply adds all these up.
(A more complete listing is available here.)
Best Answer
From that very wiki page:
So it would appear that no, you cannot hijack an Atlas in multiplayer.