HFSS radiation pattern

I try to simulate a slot antenna at THz (10^12 Hz) frequencies. In addition to the printed antenna on the substrate, a hemi-spherical lens is an indespensable quasi-optical component. The dimension of lens is much larger than the wavelength. It's impossible to put all things togther for simulation, since the computer will report not enough memory. Mine is P4 3GHz and 1G RAM.

So I splited the problem. First I run a simulation on printed antenna only. Then I want to see how the added lens changes the radiation pattern. Is there a way in HFSS I can input the incident wave from the simulation result of a solved project?

Maybe you can use FEKO or CST. You can solve large problem by them.