One of the local guys converted his Prius to plug-in. It has extra lithium ion batteries in the trunk, which tie onto the car's power bus. As far as I can tell, the car just uses the extra power without realizing it's doing it. He's had a fair bit of trouble with the battery management system; it's killed a few batteries. He recently replaced the conversion kit's BMS with a better one; we'll see how that works.
I added such a kit to our Prius from Enginer. it gave us an extra 25% boost in mileage. i too had to replace the BMS but the kit works. see Home Page for more details.