I had to buy an e-gift card from
Shop Disney... the email they send you with the gift card has clickable icons at the bottom to add to Google or Apple Wallet. I think that's the only way to add it. I've bought Disney e-gift cards from Costco, Target, and
Best Buy, but never saw the same option to add to Apple Wallet.
Once I add it, I use the Disney Gift card web site to transfer other gift card balances to the Apple Wallet card.