It looks like it's really not that difficult to retrofit a car to plug-in hybrid operation!

This Belgian company do just that, and have made a Lupo with 60 g/km CO2. Looks like they just strap a chain or belt around the output shaft of the transmission and connect it up to a motor. Maybe this stuff isn't rocket science after all!

http://www.greenpropulsion.be/product05.asp

http://mysite.wanadoo-members.co.uk/ecotech/ecot.htm