The foot brake limit switch is currently a single pole, which is NC. When you step on the brake it disconnects the 24VAC to the contactors and breaks all the motion contacts, contactors and de energizes the power contactor. The spindle switch must be placed in the stop position for the power contactor to reset. You replace the current brake limit switch with a dual pole (DPDT) snap action limit switch, the second NO pole connect the P24 from the VFD to an input programmed to freewheel the VFD. This is in the program parameters sent to you. When the 24VAC pole is broken, the other separate switch connects the programed freewheel input to P24. The VFD will freewheel as long as the brake is pressed at any position. Should you just tap the brake and release it the VFD will implement a controlled stop. When going to other more elaborate relay control system, the way that everything works is a bit different, but you still need to replace the limit switch with a DPDT. I recommend what JBOLT has outlined which is to use the foot brake as a manual system. Having done a number of systems different ways, we also overlook that there are times when you want to use the manual brake to lock the spindle for things like manual tapping, or sometimes you want some form of controlled slow braking.