create ngc edif
What exactly are you trying to do? Some simulators are require EDIF netlists which is the purpose of the tool. Since the Xilinx tool created the ngc file, there is no need to convert it to use it within the Xilinx tool.
If you want a sythesizable file, then I think you have to run the design through the "Translate" stage. This should give you an ngd file. The the tool is "ngd2edif.exe" Looking at my ISE9.1 install, they are no longer providing this tool. You would have to contact Xilinx to get a copy. According to their website, this tool was only supported up through Virtex2.