Hi T,
Could we convince you to try KMotionCNC instead? Mach3 is obsolete and we have example code for this. See SSOwithPot.c and FROwithPot.c examples.
Otherwise with Mach3 you might write a Notification C Program to read a Kanalog ADC and place the reading in a persist variable. In Mach3 write Basic code to send a notification to the plugin to upload the value to a Mach3 DRO. Then use the value to set the SSO.