Load additional "custom" fields to the advanced settings page.

pmpro-load-custom-admin-page.php

<?php
/*
Plugin Name: PMPro Custom Admin Page
Plugin URI: http://www.paidmembershipspro.com/wp/pmpro-customizations/
Description: Templated admin pages for Paid Memberships Pro
Version: .1
Author: Stranger Studios
Author URI: http://www.strangerstudios.com
*/

function my_adminpage_setup() {
    add_submenu_page('pmpro-membershiplevels', __('Advanced Settings', 'pmpro'), __('Advanced Settings', 'pmpro'), 'pmpro_advancedsettings', 'pmpro-advancedsettings', 'my_advancedsettings');
}
add_action('admin_menu', 'my_adminpage_setup');

function my_advancedsettings()
{
    // Assumes there is an '/paid-memberships-pro/adminpages' directory somewhere in
    // either the active/child theme or the parent theme.
    if ( file_exists( get_stylesheet_directory() . "/paid-memberships-pro/adminpages/" ) )
        $template_path = get_stylesheet_directory() . "/paid-memberships-pro/adminpages/";
    elseif ( file_exists( get_template_directory() . "/paid-memberships-pro/adminpages/" ) )
        $template_path = get_template_directory() . "/paid-memberships-pro/adminpages/";
    elseif ( file_exists( PMPRO_DIR . "/adminpages/" ) )
        $template_path = PMPRO_DIR . "/adminpages/";

    // Load the Admin page ('advancedsettings.php')
    require_once( $template_path. "/advancedsettings.php" );
}