Filter `pronamic_pay_mollie_payment_meta_data`.

functions.php

<?php

\add_filter( 'pronamic_pay_mollie_payment_meta_data', function( $meta_data, $payment ) {
	$data = array();

	$customer = $payment->get_customer();

	if ( null !== $customer ) {
		$vat_number = $customer->get_vat_number();

		if ( null !== $vat_number ) {
			$data['vat_number'] = $vat_number->normalized();
		}
	}

	switch ( $payment->get_source() ) {
		case 'edd':
			$data['edd_order_id'] = $payment->get_source_id();

			break;
		case 'gf':
			$data['gf_entry_id'] = $payment->get_entry_id();

			break;
	}

	$object = (object) $data;

	$json_string = \wp_json_ecode( $object );

	return $json_string;
}, 10, 2 );