Hi @Anonymous User
Bad news i am afraid you can't currently watch sky sports with a nowtv pass on a BT YouView box 
At the moment you can only watch on demand movies via the NowTV App on the YouView box.
This is going to change sometime later this year where BT & Sky have agreed to allow nowtv entertainment, kids and sports on the BT boxes.
Unfortunately it won't help you with your purchased nowtv sports pass.
More info you on what devices you can watch on nowtv.
https://help.nowtv.com/article/what-devices-can-i-use-to-watch-now-tv
