Your alpha copy won't expire, it's just a free copy of the expansion, though I don't intend to keep that particular download working after 7.0 goes live on steam. As far as supporting us, I can't think of a single alpha tester whose testing/feedback/etc contributions do not heavily outweigh $5 (or a fair bit more than that, really).
That said, I appreciate your desire to further support us.
My understanding is that Chris will be getting a batch of VotM steam keys from Valve right around the time it launches on steam, and he'll hand some to me for distribution to alpha testers.
When I get those my intent is to PM a steam key to each of the alpha testers, and those should register on steam just fine.
Once you get that, you should be all set in terms of a steam re-downloadble copy. If you would like to purchase another copy (totally up to you) the way that supports us best is to buy direct through the arcengames.com website. But buying through steam isn't a lot worse, honestly.
Anyway, here's Chris's answer on current non-steam buying methods and getting steam keys from them:
http://www.arcengames.com/forums/index.php/topic,13320.0.htmlQ: Do I get a Steam key for Vengeance of the Machine with the Indie Royale bundle?
A: Yes, but not until the expansion is actually released on Steam later this month. Once the game comes out on Steam, all you should have to do is go into your "collections" profile on IndieRoyale's website, and you'll see the new key waiting for you right there. There may also be an email to let you know when the key is ready, although I'm not sure on that and that's something IndieRoyale themselves have to arrange.
Q: What if I buy the VotM expansion through your site?
A: We can't give out Steam keys yet, but if you buy the game during the beta period, you can email us for a Steam key once the expansion arrives on Steam.
So in that answer I don't see a guarantee that if you buy a copy directly from us _after_ the beta period you'd be able to email us for a steam key. Given that, buying a copy from us now or buying a copy direct through steam after official release would probably be the most straightforward way to get a key for that copy.
Let me know if you have further questions, but I think the above covers it.