You can register it in the game just fine if you buy it anywhere, but Steam won't keep track of the key for you (it won't know you have the expansion). However, after release of the expansion on Steam in January, you should be able to register the key officially with Steam, and then it would keep track of it for you, etc. Sometimes there's a two-week delay after release before Valve gets that set up, but the only other worry there is if pirates keygen the key and register it with Steam first (which has happened to a few folks with the base game, but that's been out a long time).
To be clear, none of that prevents you from playing with the expansion no matter what, it's just a matter of having it officially registered with Steam. Hope that makes sense.