Adding a field to the history namelist is different than adding a field to the history file. If the field is already in the namelist, it can be added or removed from the history file simply by modifying the namelist called ice_fields_nml. This is discussed in the CSIM User's Guide.
The fields that are available to the history file namelist are set in ice_history.F. At the top of this module are instructions on what to modify there, in the namelist in the ice setup script, and in subroutines init_hist and ice_write_hist.