How to use wip_job_schedule_interface to set a field to NULL?

Hello,

I am trying to use the wip_job_scehdule_interface table to set some flex field (attributes) to null on discrete jobs.   Typically fields left as null in the interface table are ignored by the interface program.   In some other interfaces, such as mtl_system_items_interface, entering a single exclamation point into the field in the interface table is programmed to set it to null.  How do I achieve this in wip_job_schedule_interface?