Hi,
You want to develop a plugin of the group "hikashop" and implement the trigger "onAfterOrderUpdate" and check that the $order->order_status is confirmed and that the $order->old->order_status is created (meaning that the order is being confirmed after the payment). Then, you want to create a Joomla user account for the email address in $order->order_email (you can look at the register function administrator/components/com_hikashop/classes/user.php for how it can be done. and finally, you want to add the points to that user account basing yourself on the code of the addPoints function of plugins/hikashop/userpoints/userpoints.php
Now I can't provide the exact code. That would require a few hours of development. If you need someone to work on that for you, you can contact our partners:
www.hikashop.com/home/our-partners.html
With the information above, it should be enough.