recording audio direct from keyboard, not through midi connect

I would like to record some of my piano students through Garage band and let them play with effects, layer on strings, etc. I have a yamaha NU1 and I connect to Garage Band fine and can manipulate the keyboard in the app with my keyboard, however the tone of the piano is not great and there is no expressiveness. Can I record the audio directly from my keyboard somehow so it has more nuance? Do I need to do this through another inter app audio app? If so, which one?