In that case, you will need to create a dummy invoice and use the existing credit note to pay it off. You can then create a refund as usual from the refund tab on the bank statement import.
If you need the date of the refund to be different than the payment date then this is a little more complicated, as you would need to create a new invoice instead of a refund and give the invoice a negative unit price. You can then pay this invoice and it will effectively be a refund with the date of the original credit note (if desired) and the more recent payment date from the bank statement.
I hope this helps,
All the best