/**
 * This adds the new weight unit to WooCommerce
 */
function add_woocommerce_weight_unit_pallet( $settings ) {

	foreach ( $settings as &$setting ) {
		if ( 'woocommerce_weight_unit' == $setting['id'] ) {
			$setting['options']['pallet'] = __( 'pallet', 'woocommerce' ); // new unit
		}
	}
	return $settings;
}

add_filter( 'woocommerce_products_general_settings', 'add_woocommerce_weight_unit_pallet' );