Adding Gold Card discount to Oyster at National Rail stations

A friend has recently renewed their annual season ticket and therefore needs to add the Gold Card discount to their Oyster card again.

Previously both of us have only ever done this by finding a TfL staff member near the tube ticket machines at a main London Terminal. But their next few journeys requiring Oyster will be in the outskirts of London, on National Rail, rather than tube/overground lines. All the info we can find says that you can add the discount to Oyster at "some" National Rail ticket offices, but doesn't specify which ones.

So ... does anybody know if you can get the Gold Card discount added to Oyster at one of Broxbourne, Cheshunt, or Enfield Lock stations, at around 8:30am on a Saturday/Sunday morning? And if so, how? At the normal station ticket office counter? Or do you have to try and find a random member of staff walking around and providing help near the ticket machines? Is this likely to be hassle-free, or is it one of things where you're likely to have to shop around and try multiple times before you find somebody who's willing to do it?

This doesn't answer your question but may solve the problem: if you are going to reach a (railcard-discounted) zonal cap for the day, as far as I know it is fine to add the railcard discount after you have made one or two full price journeys, but before you have reached the discounted cap, and the discounted cap will then still apply for the whole day's journeys even though you have made one or two at the full price before you added the railcard discount.