Since the merchants ultimately want to be paid in USD (or other local currencies, but we'll just say USD for simplicity), you could start a company which takes in USD and gives back a one-to-one token for USD, which can then be either traded back and forth for crypto and associated with your Credits cards. Since this USD token - call it a "Tether?" - is 100% backed by USD, associating with a CC is basically just another debit card with a weird backend at the bank that holds the cash.