you can add the markers to your own custom script file, as long as they are in a script file they shouldn't have to be in jquery.nt-maps.js file.
There are a bunch of markers declared at the bottom of that file, so as long as you follow that pattern you should be able to add as many as you like (to your own file.) The existing ones are;
greenMarker
redMarker
pinkMarker
purpleMarker
brownMarker
blueMarker
aquaMarker
yellowMarker
That's more than 6 so might already be sufficient for your needs.
the pattern is;
var yellowMarker = L.icon({
iconUrl: '/images/marker-icon-yellow-2x.png',
iconRetinaUrl: '/images/marker-icon-yellow.png',
shadowUrl: '/images/marker-shadow.png',
iconSize: [25, 41],
iconAnchor: [12, 41],
popupAnchor: [1, -34],
shadowSize: [41, 41]
});