I dont thisk this will be possible be cause even if u give them the right to edit an order and and they try to process the credit caerd or pay by any method at that time it does not allow you to complete the payment you can try some thing like using some module that allows offline cc processing thta way you can create the order using purchase order or some thing then allow the user to edit the order during thta time process the cc i am still not sure if thta is posssible.

best solution will be you tell the user to create the order after the service is provided by u to them u can craetthe account for them then tell them what to add to cart and complete the order.

steve