IIRC in SH4 you can add labels and there is more variety in symbols, but SH3's map is pretty much limited to the names of the cities and general map colours for depths/heigths. Any "line" you want to "draw" in the map must actually be a whole terrain modification, but you could theoretically create a "hill" that is just one or two kms wide and 8000 metres high along the borders and see it in the map as a frontier line. No idea how weird that would look like if you sail closeby, but ... it can be done
