I would think you need to calculate this as a computed measure. The regular VM is obviously available, then look up the max prin and min prin and set the sign accordingly. I'm actually working on a fatigue problem right now where I have a spot with multiaxial stress, and significant compressive stress in one direction and tensile in the other. Evaluating the stress range (max(maxprin)-min(minprin) over the loading cycle has proven inadequate at this spot.
If you want this in the entire structure, you will need to run a matlab script or similar, and evaluate ascii output data files...
