bbPress.org

I had such a problem with a free guestbook hoster. I wanted to integrate it into a wordpress site.

I solved it with iMacro and javascript that parsed the sites and generated a SQL file for me which could be integrated in the wordpress database.

The problem would be getting the userdata like passwords. And a guestbook is much simpler than a forum.

But you could have the posts in this forum and I think that would be better than nothing.