How can I update a purchase code from my customer to my account?

can somebody tell me how I can keep my own account at envato and themenectarticksy (I once bought a licensed theme - the support for this has expired). Now I have a new customer, he bought the license by himself but I would like to use his purchase code in order to ask the included support for help while customizing the theme for my customer.

Is this possible?

Thanks for any good answers in advance.