The provincial Canadian government and some US City Mayors are funding the hydrogen fuel cell injection units in school buses and government vehicles to increase gas mileage and cut down on emissions such as CO2, CO, PM, NOx and HC. HFI units use a small amount of water, electrolyze it into HHO gas (two atoms of hydrogen and one atom oxygen) and inject it into the vehicle’s intake system.
As far as modding new cars already made without them. Yes tons of people have with widely varying results based on the quality of the fuel cell and their ability to adjust the cars computer and sensors to use less fuel.
Be aware reducing the fuel and keeping the same air flow will quite often cause the engine to run hot or burn exhaust valves or a piston. So the supply of HHO to take the place of the fuel removed by the vehicle computer is critical quite often. So be aware if you make these changes by mechanical means that can't be easily reversed and your HHO unit quits functioning for whatever reason you could be left on the side of the road.
So test it repeatedly after installation before taking a long road trip and maintain the fuel cell! I can't stress that enough.
Source(s):
http://www.easywaterfuel.com
http://www.globalhho.com
http://www.autobloggreen.com