@User: The mapping between symbols in xschem and device models is just a text property, so it should not be difficult to make your own symbols and point them to the RF models, or use the existing symbols and change the link to the model to point to the RF model. There is a SPICE component for S parameters but I don't know it off the top of my head; you should be able to find it in the ngspice documentation. If you want help creating or editing symbols in xschem, you can ask the developer, Stefan Schippers, on the #xschem channel.