function product_custom_field() {
global $post;
echo '
<div class="product_custom_field">';
echo 'Custom Field: <input name="product_custom_field" type="text" value="' . get_post_meta( $post->ID, 'product_custom_field', true ) . '" />';
echo '</div>
';
}
add_action( 'woocommerce_product_options_general', 'product_custom_field' );

function save_product_custom_field( $post_id ) {
update_post_meta( $post_id, 'product_custom_field', $_POST['product_custom_field'] );
}
add_action( 'woocommerce_process_product_meta', 'save_product_custom_field' );