No worries, you can get the expansion from anywhere if you buy the game from anywhere. The Steam version also includes a regular license key, so you can unlock the DRM-free version from Steam if you want. That process also works in reverse, incidentally, that the regular license keys can unlock Steam keys. Steam adds the Steam Community features for online leaderboards and online achievements, but those features are available in offline mode with all our other partners. So, you can buy it where you like, and later buy the expansion where you like! Steam will have the expansion on sale within a month from now, definitely in time for the official release in the second week of January.
