If you imported the post and thread tables as temp into your database you could potentially turn off your board and run a couple of insert subqueries that would copy the posts and threads and increment their IDs accordingly. You'd have to figure out the math so that the thread and post ID were changed correctly. The only conflict might be that I think posts are sorted by post_id and not post date in flat mode, so you might have a bit of wonkiness with post timeline for those threads.
