venca,
It is possible to run on a different clock source, but that means you will have to provide 2 crystals to the IC, one for the radio and one for the CPU. also you would have to modify the timer modules, and clock initilization and other modules that are dependent of the clock your self to make sure the system is running.
the best is to stay with the reference design and use the clk0.
Br,
Mads