As far as I know there is now import tool around for the current version of phpBB.
I've done an import of members and posts some years ago, but the db structure changed a lot since then. So I can't provide you any scripts that will help you.
If it's better to wait until 6.5 depends what should be imported to ubb.threads. AFAIK the new import tool will only convert members and post. So you'll have to come up with a custom solution if you want to import PMs, avatars etc. anyway.
