Keep in mind that purchases are locked in if you've got the Steam version. You can log into the game with or without the Steam client, but if you purchased through Steam, then any future PC purchases (like expansions) have to go through them as well.

The same is true in the opposite direction: People who bought the stand-alone PC client cannot get the expansion through Steam. All PC purchases are either locked into Steam or locked out from Steam according to how you first bought the game. (Console versions are separate, though, and can be combined with either the Steam or non-Steam PC versions. I'm not entirely sure about Mac.)