would prolly need to write an importer.. not a major job, but a 'generic' importer doesn't exist..
things like usernames would have to assume there was some sort of mapping to existing users or maybe used an anon user, who posted with a name instead..
would really be just what a site scraper would do, recursively calling from the top level and working down the layers..
maybe some rules for destination forums would also be added..
nice thing is that it is only topics/posts.. not a full blown import.. so ubbt_POSTS and ubbt_TOPICS would bear the brunt.. to a lesser degree, ubbt_FORUMS (post counts, if applicable) and ubbt_USERS (once again post counts, if applicable).. then you could use the cpanel content rebuilder to do the rest of the clean up..
can be done, but it's custom work..