The Title Block dialog ('pop-up' as you refer to it) cannot be altered, but it may not need to be.
Is 'job #' a drawing thing or a project thing? If all the drawings in your project have the same job # then it is a project thing and you could use one of the fields in the Project Details page of the Dashboard, e.g. 'Reference' which in the title block would be [Project\Reference] (see the Help topic "Substitution Fields" for a full breakdown of available fields and their references).
The Title Block dialog only contains information that is document-specific, but it sounds like job # might be a Project-level field so that would best be defined in the Project Details.
I hope I've interpreted things correctly.