BuddyPress.org

Actually, you know what you might want to do is install bbPress externally, and use whatever bbPress import scripts exist out there.
Then re-run “BuddyPress > Forum setup” when you’re ready to integrate it into BuddyPress and select the existing install option.

It might be wise to omit the forum setup from BuddyPress if you already have a large PHPBB forum post count.