Since moodle source code is available, you could modify your own installation - but then, when upgrade time comes around, those customizations would be erased, unless there was a special effort to maintain them. (Probably manually editing certain files...)
What you ask is possible - at your installation - to add a field or change a field's name means changing the
database that moodle uses, and this could result in other code not working (because it depends on certain names for fields).
But without adding fields or changing them, I can see the advantage in being able to sort based on city and country. That would not require changing the database. It would require some PHP coding.
Hope this helps - I know it isn't very detailed as to specific code changes.
