Add To Exsisting Orders: Interspire

I am looking for someone to build an addon/mod so a customer who is logged into their account can add items to their already placed order. This will also change their shipping cost and calculate the difference then taking them to pay using whatever payment options the store currently has. Customers can only add items, not delete items from their existing order. They can only modify their order if its marked pending or awaiting fulfillment. Shopping Cart: Interspire Version: 6.11 Timeline: Need this done in the next 2 weeks