Forum
Jamroom Developers
Perrie i currently have (as was previously recommended as an option) created a profile under my own user role, named the new member "Forums" and I copied the profile page url for "Forums" and pasted it as the location for where to import all my ning (non-group) main forum categories and threads. All ning forum threads and categ's were imported into that "Forums" member's forum area. In effect, the member "forums" becomes a robot member who hosts all my site's general forums. By default, in JR you can turn 'on' the ability for a member to have their own complete unlimited Forum (with categories) on thier personal profile. I then (with Paul's help) turned OFF member profiles page forums for all regular members- but "Forums" still has that function, as do Admins. Thus, my ning forums all reside within the member called "Forums" profile...but it's not too noticeable that "Forums" is actually a member. I only describe this so you can understand that setup.
But if you want threaded, then Paul's thoughts on importing your ning forums into a Group sounds like it might work for you particularly well. I am intrigued by it as well, as it sounds logical to me.