So we can actually input a very large range of functions dependent upon frequency in HFSS. We can use the Drude model for a metal, as an example. Simply place "Freq" in where f (not omega) would show up in your formula.
As for the imaginary parts of the conductivity, this appears as a term in the permittivity. If anyone knows a method to upload equations, that would be fantastic. I prepared a word document outlining this, only to realize I cannot upload it. In the meantime, I can state that since we are using solutions to Maxwell's equations that are of the form E0 * e^(iwt) the complex permittivity and conductivity are the same term, just in different forms; insert that form of the solution into Ampere's law to see.
Finally, an example input to HFSS for this model, using n, e, m and gamma instead of actual numbers,
permittivity: 1-(n*(e^2)/m)/(epsilon_0)/((Freq*2*3.14)^2+(gamma)^2)
conductivity: (gamma)*(n*(e^2)/m)/((Freq*2*3.14)^2+(gamma)^2)
Sorry about any problems with this post; first one.