How to use phased array toolbox to implement monopulse difference beamforming

As can be seen in the examples of phased array toolbox, it can implement many kinds of beamformer, but all for sum beamforming. For monopulse difference beamforming, although I can multiply +1 or -1 to the rxsignal of every element or subarray, but I can NOT multiply +1 or -1 to snapshots with no option to select, therefore can NOTimplement difference beamforming with the beamformer system object. What can I do ?