Probably not as they're owned by Sky and BT only sell the passes. You can download the NowTV app though which is available on lots more devices.
What @NigelB72 says ^^^^^
There are no plans to introduce the Now channels into the BT TV app for watching. They're in the TV guide so that you can schedule recordings on your BT TV box, but that's as far as it goes.
The Now app is available on many more platforms than the BT TV app anyway.
Ok yes, that is true. I don't know if Now TV have any plans to add support for Amazon Fire tablets, but like I say, I'm afraid there's definitely no support for Now TV content in the BT TV app and I seriously doubt there ever will be.