WooCommerce: woocommerce.php

<?php
function pixelplus_add_woo_custom_general_fields() {
	global $woocommerce, $post;

	echo '<div class="options_group">';

	woocommerce_wp_checkbox(
		array(
			'id'          => '_hot_product',
			'label'       => __( 'Sản phẩm Hot?', 'pixelplus' ),
		)
	);

	echo '</div>';
}
add_action( 'woocommerce_product_options_general_product_data', 'pixelplus_add_woo_custom_general_fields' );

function pixelplus_add_woo_custom_general_fields_save( $post_id ) {
	$_hot_product = $_POST['_hot_product'];
	if ( isset( $_hot_product ) && $_hot_product ) {
		update_post_meta( $post_id, '_hot_product', $_hot_product );
	}
}
add_action( 'woocommerce_process_product_meta', 'pixelplus_add_woo_custom_general_fields_save' );