Adds recurring donation functionality to the Donations Add On for Paid Memberships Pro: https://www.paidmembershipspro.com/add-ons/donations-add-on/

pmprodon_pmpro_donation_recurring.php

<?php 
/*
* This gist adds recurring functionality to the Donations Add On for Paid Memberships Pro.
* It adds the user-entered 'donation amount' to their recurring billing amount.
*/

//add donation amount to recurring payment amount
function pmprodon_pmpro_donation_recurring( $level ) {
	if( isset( $_REQUEST['donation'] ) ) {
		$donation = preg_replace( '[^0-9\.]', '', $_REQUEST['donation'] );
	} else {
		return $level;
	}	
	if( !empty( $donation ) ) {
		//save initial payment amount
		global $pmprodon_original_initial_payment;
		$pmprodon_original_initial_payment = $level->billing_amount;
		
		//add donation
		$level->billing_amount = $level->billing_amount + $donation;
	}		
	return $level;
}
add_filter( 'pmpro_checkout_level', 'pmprodon_pmpro_donation_recurring', 99 );