Yes that’s entirely possible!
You can even go the other way, and have Discourse manage your users and then use it as a SSO provider (what we do here with our giveaways).
I’m actually currently wondering what to do with the forum and our homepage… right now feel the quickest route is to let Discourse deal with user sign-ups because it has some cool features (such as Twitter/GH auth, log in with email, 2FA etc). Can always swap after wards…