PHP

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 );