I can't access it right now since I'm at work, but there are ways to redeem outside codes in Steam, it's possible that Valve may allow that. It's only speculation on my part right now because I cannot really confirm any information from work (stupid firewalls).

Besides, how expansions work for MMOs compared to single player games is different. Most information for MMOs is stored server side, single players are stored locally. Usually if you get an expansion for an MMO, it's simply a flag that allows you to access certain content. When everyone does the download on launch day, it's usually the same for everyone whether they got the expansion or not.