(Assuming you're talking about these "HHO" generators and hydrogen generators you hook up to your car and supposedly they give you better mileage. Otherwise, yes you can add hydrogen to the intake from a hydrogen tank, but it's not cost effective. The below refers to my initial assumption that you're talk about the HHO/hydrogen generators being marketed.)
It's a scam. There is absolutely no "science" behind the idea. If you have a chemistry/physics background and do the math on how much water you would need to get the gains they talk about, you'd be putting a 5 gallon bucket in to your car, not the little mason jar they sell. Never mind the fact that it takes near the same amount of energy to produce the H2 and O2 as you get out of it burning. The "neon light" to everyone is that these generators do not produce "HHO" as they call them, they produce H2 and O2. H (hydrogen) and O (oxygen) like to bond with one another respectively into H2 and O2. Research how much H2 you get from a gallon of water, and then research how many miles a hydrogen powered car gets out of that much hydrogen, and you'll quickly see that even as an "additive" to the combustion process there's no way you'd get the gains these "scam artist" claim.
Do the math, or ask an engineer, chemist, or physics major to do the math for you and prove to you there's no way it'll work.
It's a scam preying on those "scared" of high gas prices, and do not have technical education to know better.
Did I mention it's a scam.
