didi3r

Thanks for your help, your answer guided me to find the way to do it. I used the following code in case someone needs it:

function add_order_item_meta($item_id, $values) {
    $key = 'an_identifier';
    $value = get_post_meta( $values['product_id'], 'custom_field_name', true );
    woocommerce_add_order_item_meta($item_id, $key, $value);
}
add_action('woocommerce_add_order_item_meta', 'add_order_item_meta', 10, 2);

Regards