Tikanes(assuming you are Greek or maybe you are just a wine lover),

I will be honest with you, I am not sure if that can be done without risking the structural integrity of the car. However, I am not an engineer, but for your own safety and any of your passengers and for your investment(car), I would just save the headache and buy a trailer hitch and a trailer hitch bike rack. I just don't think it is worth the time and money to do a custom fabrication to make the opening bigger. I am also looking out for your wallet, because I'm sure this option will save you money. Finally, it will keep your hands clean too from not having to pop your quick release wheels on and off.

Ex. http://www.rei.com/product/47903116.htm