Q. Can the sector tiles be changed?
A. Yes, they're just FRMs (MSK files are used to define areas you can't traverse, such as water)
Q. Can new locations (the green circles) be added?
A. Yes, though you have to be careful, as if you add more cities than the game allows it will crash. TeamX released a patch to add cities to rectify that problem, but it involves changing Fallout2.exe (well, the patch changes it for you)
Team X's city patch can be downloaded [here] (under 'Patches').
I think there's another problem with extra cities: they won't show in the little scrollie on the right with the taped tags. They might show up actually since I never checked, but I'm quite sure they won't.
Q. Can current locations be renamed and replaced with user made maps?
A. Yes
Q. Can the world map size be increased? IE: can you add sector tiles?
A. Yes; see Editing_the_world_map