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A neighbor of mine has an electric bike, which he can peddle when the battery dies before he gets where he's going. The battery is about half the size of a moderate shoe box, and there's a motor assembly at the hub. The bike is heavy, but most of the time that's no big deal because he isn't peddling.
To do this, you need to add a generator and charge controller to his set up, plus the fairing. It gets complicated because you want to peddle for charging, and for direct mechanical drive propulsion, which means carrying some kind of mechanism for switching between the two, which will add weight. If you peddle for electricity for the motor in real time, you have to put up with the efficiency losses of the motor and generator, although it will be a simpler design.
A third option is to have a stationary generator bike that the velo plugs into. When out and about, you either go battery or mechanical direct drive. When home, you charge the battery from the stationary plant.
I've always wanted a good velomobile...
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