I am pleased you have raised this problem as I will have to do something to give the same effect. Reading through the CSLabs manual for the CSMIO IPS leads me to believe that the solution is to put 24v to the input +ve, but through a current limiting resistor. This can be fairly large, and I would suggest 3.3k ohm. then you can put the probe to the +ve input and when the tool touches the grounded probe it sends the input low and triggers Mach3. The input -ve goes straight to ground.

I hope that if this is not a safe solution then someone on the forum will shout.