I have not seen a way to add text to the map screen. You could probably change the title of the blip, which would show up in the legend on the right side of the map. But that could get messy if you have a lot of units on duty. That list is alphabetical, though, so the number would be on top and only police will see them, I assume. You could thin out some of the other blips. I have never tried to prevent blips from being seen by certain jobs while on duty. But it may be possible.
