The logic for creating phpBB users is in
/includes/modules/pages/create_account/header_php.php

You could modify that code to first check for a user by that email address in phpBB and link them together at that point.

You should also then compare the Nickname they enter in create-account vs the phpBB username and make sure that both the username and email address match. Assuming they do, sync the password from zen into phpBB, and proceed from there.