This feature will be added starting with Service Pack 3 in ISE 10.1.
With the enhancement, you can enter "X" or "U" characters in COE files for radix=2 and radix=16.
When an X or U is found in the COE file for radix=16, it will be translated to all 4 "X" or "U" in the mif file.
For example:
If "1X" is in COE file, this should be translated to "0001XXXX" in the MIF file.
If you want to use this feature with the CAM Core in CORE Generator, update to the latest 10.1 Service Pack available at:
http://www.xilinx.com/support/download/ The first service pack containing the fix is 10.1 Service Pack 3.