Steam will most likely not be offering the expansion pack for prrorder until early January, and it won't include the beta versions directly through Steam because of the way they handle that. Aside from Arcen directly, Impulse is currently the only distributor planning to offer the beta directly. However, GamersGate is also offering preorders as of today, so I will talk to Steam about potentially offering preorders sooner than we had planned, but with only the key being distributed through their platform at first (not the beta files, which you would still be able to get from us). I don't know what will happen with that, given that it will take Valve staff time to set up the store, etc, and I know they have a lot else going on at the moment. But I will float the idea with them next week.
If you buy the expansion pack directly from us, you can use it as normal with the Steam version. That does cut out the Steamworks integration (the official releases of course will not), but that's the main limitation. Thanks for your interest in the game!