RCS and S parameters are for different types of excitation.. S parameters are evaluated after exciting the structure with one or more ports and then the information at the ports (currents, voltages, fields etc. depending on the method of solution) are used to calculate it.
RCS is an estimate of the signature scattering of the body in far-field when the excitation is not through ports, but irradiated using electromagnetic wave (most generally a plane wave).. So, they are different..
In HFSS you can certainly do an RCS characterization across azimuth and elevation angles and for that you have to excite the structure with a plane-wave excitation.