One problem with compressed hydrogen might be that it will evaporate out after a week or two, if it acts anything like the liquid oxygen used for medical gas. Another thing to keep in mind with the fuel conversion is that it can be relatively expensive to put a dual fuel kit onto an existing vehicle because I believe you have to change things like the intake manifold, or carburetor if its an older one, to accommodate it. Please correct me if I'm wrong here.
The hydrogen can be stored as a metal hydride, chemically bonded to another element- at this point it can be released applying heat. As for the conversion process i am not too sure on that one, all i know is that they managed to get a 94 vette to run using hydrogen or gasoline, getting about 350 miles on the hydrogen tanks they installed.