Frequency-dependent impedance boundary in HFSS and ADS

Dear all,

I want to know how to assign a frequency-dependent "impedance boundary" to a surface in HFSS. I know I can use the variable "Freq" "e.g. Freq^2" while assigning the boundary to the surface, but what I have is tabulated values of the surface impedance Zs stored in a file "Re(Zs) and Im(Zs) vs. freq.", where Zs is the surface impedance calculated elsewhere using more accurate models.

-How to do the same in ADS momentum

Thanks