One brief statement about that and then I'll give you a tip as to where to look... you really should not store passwords as clear text.
So you might want to see if there is anything that can be done about that.
If you look in includes/usercp_register.php you will find where the 3 insert statements are run that create a new user in phpBB2. You can add a fourth insert statement to create a record in your table, and the password is - at that point in the code - available as the user entered it on the form.