On the Monotron the ribbon is simply a variable resistor. Thats why you can get non-chromatic pitches on it. On the DUO, it has a quantizer that achieves the pitch quantization, but again, the ribbon itself is just a variable resistor. So what the Monotron is looking for to control its pitch is a resistance on the ribbon controller. So if you wanted to use bottons or keys, those keys need to switch between different sets of resistors to provide the monotron with different resistances.
A much simpler solution (though perhaps it's "cheating" from a circuit bender's perspective,) would be to just use the CV pitch point on the monotron, and feed it a CV from a keyboard or controller that has a CV out. For example, a Roland A-01k, or if you have a Arturia BeatStep Pro, you can send it midi from a midi keyboard, the BSP will convert that midi to CV, and then it is a very simple modification to the Monotron to give it a CV in jack.
Having CV ins on Monotrons is a beautiful thing. If you have any CV gear such as Eurorack stuff, Korg SQ-1, Roland A-01k, BeatStep Pro, etc., you can really open up the possibilities of the Monotron, with the simple CV-on modifications that can be done.