hook into admin add order item / product on add/submit
<?php
// define the woocommerce_ajax_add_order_item_meta callback
function action_woocommerce_ajax_add_order_item_meta( $item_id, $item ) {
// make action magic happen here...
};
// add the action
add_action( 'woocommerce_ajax_add_order_item_meta', 'action_woocommerce_ajax_add_order_item_meta', 10, 2 ); 