Helge, if you do a db_dump on Postgres, you get the SQL you need to add the users in. Just make the dump, and then look through it for parties, users, and so on. It will take a little work, but I think it could be done pretty easily.
You might want to start you new installation with the acs_object_seq (is that what it's called) set to a higher number than the acs_object_seq on your current install, so that you don't overlap.
Anybody else done this?