If you have a Freesat from Sky card that will work or any other official Sky white viewing card, You just need to inform BT of the card serial number and they will activate the sports for you
Go to you BT account online..go to the BT sport of your account page section then find the "manage card"/enter viewing card number and there is a place where you put your sky viewing card number in and confirm/activate and it should then clear on sky box.
I do this a few times a month swapping with my brothers card to watch BT sport at his house then swap back to my card..works fine.