Sorry, there is currently no way to do this using only the PolySynth algorithm.
You could use one of the insert paths of the H90 to feed an external white noise generator into PolySynth to accomplish this.
Just connect the white noise to input 3, enable insert 1 before PolySynth on the routing page, and adjust the mix of the insert to 50% to allow your guitar input to come through.
You could also try this with the “Tape Hiss” parameter of Head Space feeding into PolySynth.
Let me know how it goes.
