Most National Rail stations that can issue or top up Oyster cards can do it, IIRC I've had my railcard loaded on a few times at Paddington. Probably easier to get TFL to do it, the people looking after the ticket machines can normally do it, there's a secret extra screen they can call up on the fancier new machines that lets them do it. (Used to be done at the ticket office but they're basically all closed)