It is possible to simulate power amplifiers using either bipolar or FET in Microwave Office.
S-parameters are linear parameters, so they can only be used for linear(small signal) simulation. Of-course there are some kind of S-parameters called large signal or nonlinear s parameters that are given for a specific bias and amount of input power.
For nonlinear simulation you should have one type of data that are usually used for this purpose:
1-Load-Pull data,
2-Non-Linear S Parameters
3-Non linear model, (A non-linear spice model also works)
These data either are given by manufacturer or you have to characterize the Transistor yourself.
Based on my experience, if you don't want to do an academic research and just want to make an amplifier that works good, put aside the nonlinear simulation since many models are not accurate in practice. Just Design a good amp using s parameters. make the amp and tune it out for the high power application that you want.