i have a question about origin games.

You can add it as a non-Steam game. You will have overlay and you could launch it through Steam. Nothing else.

(There are some that can be activated on Steam, such as Alan Wake)

EDIT: I completely misread it, yes you can do that.

EDIT2: I swear there were two comments above me :S

Yes, you can. It makes no difference :D

Steam codes for EA often work on Origin apparently, but never the other way around AFAIK.

If you have the game code registered on Steam and Origin, you can play via either client.

Until EA sends their soldiers to take over Valve at gunpoint and forcibly delete their games off of your account, they'll remain on your Steam account. So, you know, about a 50/50 chance

For me it worked fine with Dead Space 2, so I guess it will work for Crysis 2, too.