Add custom fields to XML for the WooCommerce Customer/Order XML Export Suite

gistfile1.php

<?php
function add_custom_fields_to_xml( $order_data, $order ) {

    $order_data['BillingCompanyID'] = ( isset( $order->order_custom_fields['billing_company_id'][0] ) ) ? $order->order_custom_fields['billing_company_id'][0] : '';
	$order_data['BillingCompanyVAT'] = ( isset( $order->order_custom_fields['billing_company_vat'][0] ) ) ? $order->order_custom_fields['billing_company_vat'][0] : '';
	
	return $order_data;
}
add_filter( 'wc_customer_order_xml_export_suite_order_export_order_list_format', 'add_custom_fields_to_xml', 10, 2 );