the device class / device driver is only necessary if you want to interact with your hardware drivers (e.g. GPIO, ADC, CAN, ....).
If you want to insert push buttons like a keyboard, then you can adapt the main loop and insert the button press as a key event.
Let's assume you have two hardware buttons: One for left and one for right navigation. Within your main.c you can adapt the function that translates the standard keyboard codes into key events: When your left button is pressed you can insert CoreKeyCodeLeft and when your right button is pressed you can insert CoreKeyCodeRight.
The advantage is, that you can easily test your application within the Prototyper by pressing the corresponding keys on the keyboard (e.g. left / right).
I hope this answers your question.