Someone's tried it before: http://www.nutritionask.com/
With a bunch of widgets, changes in the design. And now you can sort by upvotes too so it's probably much more possible to do a Q&A style site than it was a month ago when you first asked 🙂
Is it possible to integrate NodeBB into a cms like keystone, especially sharing the user authentication/management with a cms?
You'd probably have to write an SSO plugin to share authentication. @julian has a ton of SSO plugins that you could use as a sample in writing your own one
As far as user management... somebody was asking for a WP user management plugin. Kind of like if you upgrade a user's privileges in WP, it updates it in NodeBB as well. It's doable but the client didn't end up going for it so we never ended up building it