Jason,
YAF 1.9.3 supports ASPNET Membership, which it sounds like what you're using. Is your YAF site installed yet?
Yes, this sounds doable. It's a matter of configuring the web.config (in 1.9.3) to use your existing database. I'm an Independent and have done this before, though I'd need to find a window to make this happen for you. Feel free to ping me offline if you want to discuss further.
Regards,
Dave