DLC ON Steam
 

I have a question. I noticed they have DLC for this game on Steam. However, it says that to use such DLC, I must have MOW: AS base game on Steam. I have the base game, but it is through Amazon Downloader. Can I just add the game to steam library and use it or does it have to be bought specifically from Steam to use the DLC

Yes, you can use your gamespy cd key that u got with the game and register your product with steam. steam>library>games>activate product on steam.