The All Products for Subscriptions plugin allows you to add subscription plans to your existing products. This allows the customers to optionally subscribe to certain products. With this plugin, you can offer subscription plans not only on the product page but also in the cart, which allows the customers to add the plans right before checking out. The customers can also add products to their existing subscriptions, which allows them to manage their subscriptions easier. In addition, you can offer discounts to those that choose to have the subscription plans.
Please note that this plugin requires the Subscriptions plugin.