How can I add funds to Apple Pay Cash without a debit card? For example, can I add funds to Apple Pay Cash from my Apple ID funds? Or my credit card?
According to the only answer to this related question, it is possible to add funds to Apple Pay Cash from a credit card, which incurs a 3% fee. How can I do this?
The linked post is almost 4 years old - a credit card  is not accepted currently. There is also currently no way to add money from your Apple balance or other external sources.
Apple Support:

You can only add money from a supported United States debit card or prepaid card that's in Apple Pay.