Hi Colin,
Sadly you can’t add anything to a 60+ Oyster card. It’s an off-peak season ticket, but because its validity sometimes depends on which train you use, the system can’t know when to use credit and when the ticket is valid.
When travelling beyond the validity of either a 60+ Oyster or a Freedom Pass you’ll need a paper or smart ticket from the boundary station. The train doesn’t need to call at that station. Sometimes a ticket from boundary of zone 6 to your destination will be better, particularly when there are more than one route that the train could take.