TFL allows you to add a National Railcard to an Oyster card. However, most people use a bank card either with the physical card or via an NFC payment system.
Is there any way to add an existing railcard to a debit card or credit card or Google Wallet?
You cannot.
As of November 2022, contactless (bank cards or NFC devices) support only extends to a "pay as you go" model for a single adult fare with daily and weekly cap.
For railcard discounts, student/young persons discounts, travelcards, season tickets, etc., a registered Oyster card is required.
