Importing Activity Codes from Excel to P6 - blanking problem

I work in a P6 environment with multiple imports happening constantly from many people. Sometimes my activity codes get overwritten to an earlier point in the project development. I would like to keep an Excel export file handy to re-import the "true" values when these overwrites occur.  I realize that an import will not modify a particular activity code if the code is not in the dictionary.  This is good. My problem is in cases where I want to 'blank' the code, e.g., the code field has a value of "CA" and I now want it blank ("" or null). It seems that (as a business rule?) P6 will not allow a blank value in an Excel file to overwrite a populated code field.  I can think of a couple workarounds, but they would be quite laborious. Any suggestions would be appreciated.  Is there a magic, undocumented value that would tell P6 to make it blank?