I have an Oyster annual season ticket, zones 3-5. I want to add zone 6. Has anyone managed to do this?

This is my track record so far:

Oyster website says - phone our customer line or go to underground station.
Phone customer services - they say go to underground station.
Go to Wimbledon - they say 'we're not an underground station'.
Go to East Putney - they can't do it.

Is there a magical ticket office somewhere on the network that can do all I wish?