Yeah, you've seen plenty of companies add plug-in capability to the Toyota Prius. Generally speaking, the battery pack is enlarged to allow for greater range when operating in electric only mode and a charging system is added which allows the vehicle to be charged by plugging into a standard outlet. Of course, this is all harder than it sounds! The standard Prius is only capable of charging its batteries using the on-board internal combustion engine or under regenerative braking and is equipped with nickel metal hydride batteries. The standard batteries are usually swapped for a new pack of lithium ion batteries, which carry a greater energy density than the older nickel based batteries.
Lithium Technology Corporation has gone one step further and used a new type of lithium battery that they have developed. The composition of the batteries is different and they are known as lithium iron phosphate cells, and are considered to be safer than the more common types of lithium ion batteries. These batteries are not completely new, but have never been made in sizes this large before. With these new batteries, LTC was able to demonstrate 125 miles per gallon equilavent with their converted Prius. No price was mentioned in the article, but I expect that the batteries added a substantial amount of money to the cost of the car.