said by cdru:
Worst case, a plumber or the city could shut off the line to the house so that one could be added. I've also seen a valve replaced added without shutting it off at the street by freezing the line to plug the water flow. Dry ice and liquid nitrogen have been used. Probably not something you'll want to do without experiencing though as Bad Things™ can happen if you mess it up.
Sidewalk valves have a tendency to get paved over during sidewalk replacement in NYC. Rather than use dry ice (or better yet, dry ice, a garbage bag, and alcohol), you can the correct tool »www.ridgid.com/Tools/SF2500-Pipe-Freezer
. Warning, price tag says you will be renting it.