Mike Jolley wrote a Woocommerce plugin named Product Addons that allows global and per-product addons as free or paid upgrades. The plugin provides several template files, but they can’t be overridden in the typical Woocommerce manner.

Instead of placing template override files in the woocommerce folder in your WordPress theme, place them in a folder named woocommerce-product-addons in your WordPress theme and you’ll be good to go.