Theoretically, you could modify the Your Account module easily to accomodate for a registration through the iBill credit card option. You could
proceed along the following lines (see Replacing registration with iBill credit card option):
The iBill system essentially works by users entering information into the iBill script. The script would then connect to the web server and create an account in an .htaccess file (Section 25.4) in the PHP-Nuke root directory. When a paid user account would expire, iBill's server could then remove the user account from
the .htaccess file.
With iBill controlling the .htaccess file, there would be no need for a Registration option in PHP-Nuke. The only thing that would be needed would be to change the
code in block-Login.php, perhaps replacing the Register link with a link to a static html page, where the iBill sign up scripts would do the trick. Or you could just put the iBill code in a block and
remove the Register option.
There is no ready-made solution for this scenario yet, but the procedure outlined above points to the right direction.