look at the available filetypes available for export from auocad, the look at the types available for import in HFSS v. 11 and see if there's any that match.
for hfss v.15: sab, sat, sm2, sm3, anstgeom, xls, dxf, dwg, exp, model, CatPart, CatProduct, gds, iges, igs, nas, x_t, x_b, prt, asm, step, stp, stl, sld are the import options.