Is it possible? What I mean by that is can I add CoD4 which I bought at the store and add and save it to my Steam account so if I logged on to a different computer it would still be there? Kinda like a back-up copy? I really don't mean to do anything illegal.
Nope. Only thing you can do is add a shortcut to steam.
That's to bad. :( Thanks for the quick answer though. :)