Water pump as air compressor?
Yes, you can use a water pump to indirectly pressurize a gas with water pressure if you use a water tank with an internal air bladder. First, you drain the water from the tank. Second, you fill the air bladder with your gas through the Schrader valve. Third, you pump water into the water tank to the pressure you want your gas. Fourth, your gas in the air bladder is now the same pressure as the water in the water tank and you can remove it from the tank. 

The opposite can also be done by using air pressure to create water pressure if you ever have an air compressor and need to make water flow instead.

Most tanks have a maximum rating of 100-120 psi pressure, and hydrogen fuel  tanks operate at a working pressure of 5,000-10,000 psi, so I think you'll be fine using a water tank to indirectly compress hydrogen to 100 psi. Direct contact with an electric pump is the last thing you should do with hydrogen, so use this technique and you'll be safe. 

Alternatively, you could replace the water pump with an air compressor like a bicycle pump and achieve similar results using this method. I would use this or a combination of both water and air in the water reservoir if your small water pump can't create the necessary pressure. Essentially you're using one bladder to pressurize a second bladder, which works because they're both trapped inside of a container with a fixed volume.