You might be able to achieve this by using the buttons to connect different resistors from + to an input. You would then need to read in the analog value and check to see if it is a certain range.
If you just need to connect two buttons to each input, you could wire a 100k resistor permanently from the input to +, this will cause the analogue input to read about 128 (there is a 100k resistor connecting the input to ground, or -, on the Crumble). You could then connect one button from the input to – and the other button from the input to +. The program is slightly simpler because you can check if an analogue input is below a value for the first button, or above a value for the second.
I’ve attached a simple program for one input.
I hope this makes sense! Let me know if you need a better explanation 🙂