Unfortunately, while not impossible it's nearly that way. There is no way to do it with the importer without duplicating users, etc. The only perceivable way would be to actually recreate all of the posts by hand and then go through one by one and reattach them to the proper userid. It would be a lot of work, and you'd need to know your way around the database as well.
Not the answer you wanted to hear I'm sure.
