Unless you know at least a little PHP, and also are very familiar with php-nuke AND the phpBB ported forums, MySQL database, the different sql queries you would need to add, the different fields to the form and so on...

unless you know basically the above, then it's gonna be rather dificult. It is "possible" though.
Have you looked at www.nukescripts.com
They have a Your Account module for PHP-Nuke and PHP-Nuke with phpBB port Forums. IT has a few more features in the registration process.

mikem