First no country has its embassy in its own country. A country's embassy or consulate is always located in a foreign country. Second,the immigration office where the extension is done does not work on Saturdays. Third, there are several locations in Doha where you could go to. There is an immigration office a Al Messied where you can go. They are very efficient and you may be able to finish your work within 30 minutes of your arrival there. You can find out about other locations of the immigration office from your mandoob or your colleagues.