If you have an appropriate driver that accepts step and direction signals, such as a Gecko G320 or G340 and the servo has an appropriate encoder mounted on it. Then the servo can function as a "step/dir" spindle motor. The problem with Mach3 will be if it can generate the step and direction signals for the spindle at a sufficient rate to turn the desired rpm. The other problem you will face is writing the gcode to both keep your spindle turning and making the cuts you want.
If this is just to have a means of indexing the spindle, then it may be reasonable. If you are wanting a 3000rpm spindle then it is probably not a good approach.
So the real question is what are you trying to achieve and is this the best way to do it?