splashingpixels.com on "[Plugin: WooCommerce - excelling eCommerce] Adding permanent custom themes"

Have you looked into adding custom meta with custom meta boxes? Once you add that meta box, it will appear on every product you create and in there you add your custom meta value. For more information on how to do this refer to the WP Codex here -> http://codex.wordpress.org/Function_Reference/add_meta_box