At the Detroit auto show next month, one of the most highly anticipated new vehicles will be Toyota Motor Corp.'s third-generation Prius hybrid. It features a roomier interior and better gas mileage than the current model, the best-selling hybrid in the world.

But even these advancements aren't good enough for Daniel Sherwood and Paul Guzyk. The two green-minded mechanics have been modifying Toyota's Prius hybrids, which get an impressive 50 miles per gallon, and converting them into plug-in electric vehicles, doubling the...