At this time, we have not performed any compatibility testing with the WooCommerce Subscriptions functionality. As a result, you may encounter issues using Product Add-ons with the WooCommerce Subscriptions. Plus since WooCommerce Subscriptions is a Paid plugin we don’t have easy access to the code to verify and test.
Our recommendation, try a testing staging environment with WooCommerce Subscriptions and Product Add-ons plugin. If you encounter compatibility issues, feel free to reopen the forum post and add details here. We will evaluate and understand if we can develop the needed compatibility.