If the case fields provided by FogBugz don't quite serve your needs, use the
Custom Fields Plugin to add a new one in a matter of minutes. The field can be any of several types including text, drop-down, and date/time.
Once the custom field is created, you'll be able to edit the new field in the case view, see a new column in the grid view, and even filter results by possible field values.
Are custom fields documented in the data dictionary? Where is the data in the custom fields stored and how do I join from that to the bug table? Also, if I use a drop down custom field, where are the value choices stored?