Hi, the Prius has the option to go in Electric mode only BECAUSE the base model sold in Europe HAS that button already fitted.
When the Prius is sold in USA Toyota decides to REMOVE that button.
The aftermarked mod is ONLY retrofitting a button to some already exitent contacts hidden under the dash.
Other cars have different behaviour/construction and there is really nothing to do; retro-fitting a push-button is easy, making a car to accept that button (from scratch) is a complete new question.
Hope this info answer your question ;)