Many years ago, I posted the same question when I was using version 7, http://forums.adobe.com/thread/316088. Now I'm using FrameMaker 11 and need to do the same thing: add a new symbol to the standard list of symbols. I followed the instructions in the old thread step by step to the letter, yet the new symbol does not appear in the list.
Maybe I didn't use the correct file. The path I used for menus.cfg file is