Yes if you have your own expressjs app then you should be able to use the templating engine we use. It is a separate project here https://github.com/benchpressjs/benchpressjs.

Do note that nodebb is a forum platform, not a CMS. I've seen some awful attempts at trying to shovel a CMS into a forum platform in my time. SMF springs to mind.

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 https://github.com/julianlam?tab=repositories 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

Here sir, take my vote.

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

Can we use this plugin to add announcements, reference to rules, or any html specifically to the login page?