You can't save directly in the CS format from inside FSC, but you can save it as FSX plan and then import it into the CS FMC with the REQUEST function.
http://img833.imageshack.us/img833/356/requestk.jpg


Quote:

On the real aircraft, selecting this prompt sends a datalink route request. In the simulated FMC this prompt opens the SELECT CO RTE page listing all previously saved routes. You can navigate the list pages using the NEXT PAGE and PREV PAGE keys. Selecting any route from the list loads this route into the FMC. The FLTSIM FPLS on the bottom of SELECT CO RTE page opens IMPORT FS FPL page. This page lists all flight plans (.FPL files)  created and saved by  the Flight Simulator built-in flight planner. Selecting any entry imports the corresponding flight plan waypoints into the FMC.