it wouldn't be easy, you could probably make a script to do it, but what happens when the same user is on both forums?
That depends on whether it's the same person, or different people using the same username. The only check wold be the email address. If the username and email both match, then I'm importing the posts and updating the user's postcount. If not... well I only have one case like that, and the duplicate user has 0 posts
I've been working on a script to do the merger, but it's tricky. Topic ids and post ids of the imported data have to be altered, amongst other things. So far I've got the users, topics and posts to import, with new (incremented) ids, and correct data entered into last_post for each topic. But something isn't right yet. When I go to a forum and click an imported topic, e.g. 'id=892', I get an error message about an outdated link. But in the database, the topics table does indeed have topic id 892 in there...
Back at it tomorrow I hope.