Okay.
I would strongly suggest you do this on a development machine firstStep One. Make a backup HTML copy of your registration form.
Step Two. Make a backup of your MySQL PHPBB database.
Step Three. Disable new registrations.
Step Four. Create a page inside AM that includes (ENglish include, not PHP include) that HTML form.
Step Five. Process and insert new row/relevant data into PhpBB.
Done.
Further questions may be asked and I take no responsibility in answering them since I'm an Invision Board guy, not a PHPBB guy.
PS: If you wanna switch....to anti-bot forum software

Just gimme a ring