Can I log onto an existing Tribes: Ascend account through Steam?
Solution 1:
The assumption from this article is that you'll be able to tether your existing account to your Steam account. I'm not 100% sure as I personally don't play Tribes. However, my experience with Borderlands (which is also run by GameSpy) tells me that you'll be able to link your account.
Solution 2:
Yes, you will be able to; I've done it.
The Steam version of Tribes still uses the HR authentication.