As it stands, the machine's limit sensor input looks like a logic input pulled up to 3.3V via about 8.5kΩ. The 0V connection is the wire nearest the machine.

The microswitch is wired normally closed, and has a 10kΩ resistor across it so that when contacted its resistance rises to 10kΩ.

Thus the input looks like a high-impedance logic input, active high, that rises from 0V to about 1.8V 3.3V when the limit is reached. This is what we need to emulate with our combined sensor.

Rather inconveniently there is no third wire with 3.3V or 5V on which we could use as a supply.

(Edit: ignore the stuff about a 10k resistor - I think I added that at some point, and messed up, it was a bad idea. It's gone now.)