IMHO, if you knew why you needed a fuel cell- you would already know how to set it up. Why do you need it?, if you don't mind.

You can probably just tee in a fuel cell, in-line pump, and then the line into the main feed. Just have a switch for the cell pump. Your fuel regulator will maintain the pressure. You can probably find a one-way valve and put it in-line with the cell line to avoid pressurizing it from the stock pump, your in-line pump might have that built in. This is just a guess.