Add a feature for Joomla K2 shop so a buyer will be added to a specific usergroup

We need to be able to add a user in to a user group automatically depending on what product they buy in the K2store in our Joomla site. It is aJoomla 3.4 site with K2 v2.6.9. A dropdown in the admin section for each product (item) needs to be visible for admin to chose a user group from Joomla User Goups which will assign the buyer to that user group after successful check out.