It isn't any different than using custom graphics in levels, really.
The graphics used by the overworld can be changed in the
dialog. As you can see, GFX1C, 1D, 8 and E are the graphics used in every map. Some overworld graphics come with files named after GFX files rather than ExGFX because usually you'll want to replace them globally; but nothing stops you from inserting these or any graphics as ExGFX and changing the loaded graphics in each individual submap. Note that you also have two extra unused slots (FG5 and 6) if you want to just add to the original graphics rather than replace.
You can see the graphics loaded in the
dialog (each of the first 3 pages is 2 files), and just use them directly from there.
Also note that the overworld does not have a button to extract GFX/ExGFX; you'll need to use the buttons of the level editor for that (
) and then reload the overworld graphics with