You can output the center of mass (COM) however you want. Note, though, that it will report the COM for every step of the simulation. If you only want it at the start or the end, you will have to extract that from the simulation results.
If you can use the final step of the simulation, a simple way to get that value is to limit the length of the measured results to 1. It will then only save the results from the last time step.
--Steve