Have you thought about Wheel Adapters rather than drilling another set of holes in your wheels & weaken them.
Google is your friend - https://www.google.co.uk/?gws_ ... adapters+4x114.3+to+4x100
You will probably have to get your arches rolled as well by the sounds of it, also you might be able to change the hubs on the car for different size ones but thats going to be expensive. Another option is see if anyone else has done a wheel conversion your thinking of doing on an owners forum. You never know someone might have already done it for your car.