Cayenne has certain device drivers already baked into the installation. These sensors can be added no problem as long as they are wired correctly.
Unfortunately, we have not opened ability to read UART or Serial on Pi. After Arduino integration, we’re going to work on allowing you to bring on any sensor with the Pi. Unfortunately, I don’t think it is possible to combine xrf module with Cayenne at this moment. But our goal is to allow anyone to add any sensor to Cayenne.