It wouldn't be Spriteset_Map, although you might also have to modify this one; the class which manages tiles is called Tilemap, and it's a core class.
It is possible to modify it; I suggest looking into the .chm file that comes with the program and see what you can figure out. Tilemaps are generally the most common ways of dealing with graphical representation (for the world) in 2D games, but Enterbrain's is but a specific implementation; I'm sure many others would be good, or better, or a lot different. Hexagonal tiles, for example, would be neat; but remember the format of the tilesets would also be different.
If you're modifying a core component like that, you're going to do a lot of work both on the scripting and graphical side; I wouldn't recommend it, but if it's necessary, you gotta do what you gotta do.