Welcome to the forum.
I am not clear on exactly what you are trying to accomplish. It seems that you are trying to Save a Project in CS 6, that will open in CS 5, but I might just be mis-reading. Can you provide more detail on what your starting point is, and what you are trying to do?
Thanks, and good luck,
Generally you can't go backwards. You may have to continue on in CS6 only.
FCP XML for simple projects, as last resort...