WP: Woocommerce add new custom field checkout

gistfile1.php

//example

function add_custom_stock_type() {
?>
<script type="text/javascript">
    jQuery(function(){
        jQuery('._stock_status_field').not('.custom-stock-status').remove();
    });
</script>
<?php   
    woocommerce_wp_select( array( 'id' => '_stock_status', 'wrapper_class' => 'hide_if_variable custom-stock-status', 'label' => __( 'Stock status', 'woocommerce' ), 'options' => array(
        'instock' => __( 'In stock', 'woocommerce' ),
        'outofstock' => __( 'Out of stock', 'woocommerce' ),
        'onrequest' => __( 'Disponibile su richiesta', 'woocommerce' ), // The new option !!!
        'onstore' => __( 'Disponibile in negozio', 'woocommerce' ),
    ), 'desc_tip' => true, 'description' => __( 'Controls whether or not the product is listed as "in stock" or "out of stock" on the frontend.', 'woocommerce' ) ) );
}
add_action('woocommerce_product_options_stock_status', 'add_custom_stock_type');
function save_custom_stock_status( $product_id ) {
    update_post_meta( $product_id, '_stock_status', wc_clean( $_POST['_stock_status'] ) );
}
add_action('woocommerce_process_product_meta', 'save_custom_stock_status',99,1);
function woocommerce_get_custom_availability( $data, $product ) {
    
    $stock_status = get_post_meta($product->id , '_stock_status' , true );
    
    switch( $stock_status ) {
        case 'instock':
            $data = array( 'availability' => __( 'In stock', 'woocommerce' ), 'class' => 'in-stock' );
            break;
        case 'outofstock':
            $data = array( 'availability' => __( 'Out of stock', 'woocommerce' ), 'class' => 'out-of-stock' );
            break;
        case 'onrequest':
            $data = array( 'availability' => __( 'Disponibile su Richiesta', 'woocommerce' ), 'class' => 'on-request' );
            break;
        case 'onstore':
            $data = array( 'availability' => __( 'Disponibile in Negozio', 'woocommerce' ), 'class' => 'on-store' );
            break;
    }
    return $data;
}
add_action('woocommerce_get_availability', 'woocommerce_get_custom_availability', 10, 2);

function my_woocommerce_notice() {
?>
<div class="error notice">
    <p><?php _e( 'Il plugin WooCommerce è stato modificato nel core. Si prega di non aggiornare il plugin, se non da personale esperto. (file modificato class-wc-admin-post-types linea 441)' ); ?></p>
</div>
<?php
                            }
add_action( 'admin_notices', 'my_woocommerce_notice' );