To apply PSS the circuit must have a periodic solution and the period must reasonable relative to the frequency of the drive signals. For this reason, fractional dividers are usually not appropriate for PSS. Particularly the newer delta-sigma dividers. Here the period is very long relative to the input clock frequency.

However, there is another way. You can characterize the phase noise of the individual stages and use these to predict total phase noise. This process is described in "Predicting the phase noise and jitter of PLL-based frequency synthesizers" in http://www.designers-guide.com/Analysis/.

-August