Q: How to add an extra bank card

I assume you are asking how to add a card to Apple Pay.

Settings > Wallet & Apple Pay > Add Credit or Debit Card

If instead you were asking about adding a card to your Apple ID / iTunes, then you can only have one card at a time registered there.

To change the card registered to your Apple ID:

Settings > iTunes & App Stores > Apple ID > View Apple ID

Sign in there and change the card -- (Payment Information)

Your bank should be able to help you ... if you're sure you're being asked for that three digit security PIN on the back of your card.

Glad that helped.

Best of luck, and enjoy your Apple gear!