I did see a previous post with the same request and its seem SmartRF Studio is restricted to the pre-programmed frequencies although you could try and set the registers manually in SRF Studio. I had the same problem with a CC1110 at 24Mhz, in the end I manually calculated the values and verified against the CC1111 values in SRF Stuido. You can also use the CC2511 values from SRF studio in offline mode as a starting point.