Visa Extension while in China

I am travelling to Beijing for the World Championship as a tourist and would like to visit Xian and Shanghai after the games end. My visa is for only 11 days and I would like to spend another 8 days touring Xian then Fly out of Shanghai back home. This flight was booked before my entry. Is there anyway to get an extension to cover the rest of my planned travel. Where would I apply in Beijing?