There are some companies that will make plug in conversion for the 2004-2009 Prius, I don't know of any for the 2001-2003 model still, you might ask. A place to start is
http://hybridcenterma.org/ . The problem with using either Prius as a plug-in is that the transaxle is partially lubricated by the engine rotating. If the engine doesn't run often enough... Of course, there are ways around that, but that will be in addition to the effort put into making a plug-in.
Another problem is how will you maintain the battery. If you fully charge and discharge a NiMH battery it will last for about 300 cycles--not particularly cost effective. The Prius gets its very long battery life by never fully charging or discharging the battery. You'll need to do something similar.
100mpg prius conversion?
Linear Mode
