You should be able to add it using ExifTool Custom Columns
If nothing other than the Windows Properties dialog displays the field, it seems like it'd be better to move it into another field, else it's still going to be a problem in every other program. (Either by asking the people who make the tool to change it, or batch processing the files with something like ExifTool.) But if that isn't an option, the ExifTool Custom Columns add-in should let you add almost any field to Opus.