Add Custom WooCommerce Order Status

function register_custom_order_status() {
    register_post_status('wc-custom-status', array(
        'label' => 'Custom Status',
        'public' => true,
        'exclude_from_search' => false,
        'show_in_admin_all_list' => true,
        'show_in_admin_status_list' => true,
        'label_count' => _n_noop('Custom Status <span class="count">(%s)</span>', 'Custom Status <span class="count">(%s)</span>')
    ));
}
add_action('init', 'register_custom_order_status');

function add_custom_order_status($order_statuses) {
    $new_order_statuses = array();
    foreach ($order_statuses as $key => $status) {
        $new_order_statuses[$key] = $status;
        if ('wc-processing' === $key) {
            $new_order_statuses['wc-custom-status'] = 'Custom Status';
        }
    }
    return $new_order_statuses;
}
add_filter('wc_order_statuses', 'add_custom_order_status');