Yes and no. As pointed above is not the most efficient method but on the other hand...
Betavoltaic batteries are not very efficient in terms of output, so probably you cannot power the android on them alone. Massive, after all, would probably mean "too massive to load into an exoskeleton", as there is a good reason betavoltaic haven't seen much use beside buildings (space probes don't use betavoltaic, is more like a peltier effect on a very radioactive source; you don't want this on your mech). In this case, the hydrogen cell would be like an accumulator that drives extra energy into the exoskeleton (and that could be refueled externally before use), the betavoltaic acting like an auxiliary power source that while it's not in use can regenerate small amounts of fuel, not enough to make a difference in action but let's say you find yourself stranded from the group, enough to ensure that you can go back to the base if you let it work overnight.
For any of this to be useful, it would make more sense if your exo has access to plenty of water (let's say a very humid environment) so the system can get water from ambient via condensation, instead of a closed system. A purely closed system would probably have less problem using batteries or supercondensers, unless the water or the hidrogen is used for something else (exos are equipped with hydrogen torches, or they have rockets, the water is used in life support systems, etc).
So, without having much information about the context where they will be used I will say that (sorry for yodaspeak) feasible... it is, sense does not make much.
If you provide better context I may be able to expand the answer with better information.