Problem Description: In ISE Project Navigator, under Processes for Current Source -> Right click on Synthesize -> Process Properties -> Synthesis Options, there should be an option called Keep Hierarchy. However, it does not seem to be available.
This option is not available if the Property Display Level is set to Standard. Go to Edit -> Preferences -> Processes -> Process Settings. Set the Property Display Level to Advanced and the Keep Hierarchy option will be available again.
Please see also (Xilinx Answer 11088)