Modify Subscriptio plugin for Wordpress

I need to modify an existing plugin called ' Subscriptio '. Basically Subscriptio is a plugin which allows you to charge re-occurring subscription payments via woocommerce on a weekly, monthly or yearly bases. At the moment the plugin only takes the payment from the user on the date they purchased the subscription. What I need you to do is, to modify the plugin so that the user can select the date they want their subscription to be taken. Here is an example: 1. I have a product which has a yearly subscription. 2. When on the product page, before the 'add to cart' button there should be a dropdown box which allows the user to select the date they want to be billed each year (example 10th December). 3. Now each year on the 10th December the user is billed via the plugin. This is the plugin [login to view URL]