The Nicolson-Ross-Weir method for deriving effective constitutive medium parameters is quite frequently used. The method allows one to derive these parameters from scattering parameters, which may be easily obtained in HFSS. You can read up on the methods in their papers:
A. M. Nicolson and G. F. Ross, "Measurement of the Intrinsic Properties of Materials by Time-Domain Techniques", IEEE Transactions on Instrumentation and Measurement, Vol. 19, No. 4 , 1970
W. B. Weir, "Automatic measurement of complex dielectric constant and permeability at microwave frequencies", Proceedings of the IEEE, Vol. 62, No.1 , 1974