What is the best way to add forums to an existing app?

I hope this is the right forum for this question. Apologies if not.

I have an existing hosted web application, and I would like to add forums to it. If possible, I would prefer to use one of the pre-packaged forums applications, such as Community Server, DNN, etc., rather than having to write a forum myself.

What is the best way to do this? I have considered installing Community Server over my existing application, then somehow integrating my application into the community server pages or directory. This seems suboptimal because I'm trying to add forums to my app, rather than integrate my app into CS, and I'd be afraid I'd lose the look, feel, and control flow that already exists in my app. I have also considered installing CS or DNN to a sub-application, but then I don't know how to achieve single sign-on (so that signing in on my primary application also results in being authenticated in CS.)

Is there a better approach, e.g. a public domain forums application that I can manually integrate into my app?

Some of the starter kits have forum code you can use, and the Community Server forum code (original version) should still be available somewhere. Do a Google search and you'll find plenty of forum code options.

Jeff