You can Insert Circuit Design. That supports non linear models (like transistor switches). And then you can directly connect it to HFSS full wave EM models. But this will only allow you to see circuit parameter responses (S-Parameters etc) of your final integrated design. If you want to see farfield/nearfield performances of your design, try Option 2.
You can characterize your transistor states fully and implement some kind of Freq dependent (or any other variable dependent) source impedance/loads. This can be done using "Design Datasets..." feature. The transistors will not actually be in the design, but the performance of transistors will be included in your results.