How to add custom status for Woocommerce.

functions.php

<?php
// The following code adds the status "Reserved".

/**
 * Register new status
 */
add_filter( 'init', function () {
	register_post_status( 'wc-reserved', array(
		'label'                     => 'Reserved',
		'public'                    => true,
		'exclude_from_search'       => false,
		'show_in_admin_all_list'    => true,
		'show_in_admin_status_list' => true,
		'label_count'               => _n_noop( 'Reserved <span class="count">(%s)</span>', 'Reserved <span class="count">(%s)</span>' )
	) );
} );

/**
 * Add to list of WC Order statuses
 */
add_filter( 'wc_order_statuses', function ( $order_statuses ) {

	$new_order_statuses = array();

	// add new order status after processing
	foreach ( $order_statuses as $key => $status ) {
		$new_order_statuses[ $key ] = $status;
		if ( 'wc-processing' === $key ) {
			$new_order_statuses['wc-reserved'] = 'Reserved';
		}
	}

	return $new_order_statuses;
} );