Here in PHL, all ground vehicles have an ICAO code that converts to N7xxXB. Most of them even ident their actual vehicle number. Airports ops are A9, A10, A209, A219, A229 and others. There’s also MEDIC30 and F8 and F10 of ARFF. I forget what all the snow removal trucks use, but they all have ADS-B and show up on the map.
I modified my map to use GND as the aircraft type for any “aircraft” with XB suffix that is on the ground. Then I also used FlyingPete’s custom icons (http://discussions.flightaware.com/ads-b-flight-tracking-f21/some-custom-svg-plane-icons-t37783.html), which includes a pushback tug, and linked GND to the pushback tug.
It all works pretty well. But this is all for PHL, so it may not be the same at your airport.