The only way that you would be able to register a PayPoint voucher on your account would be if the voucher you have received is similar to a credit or debit card. I know that 3V is available to be used towards purchases on PayPal but I'm not 100% sure if PayPoint is compatible.
If you check the voucher that you have received and see if it has a card number, expiry date and CVV2 number. If it doesn't, then you wouldn't be able to register it.
