I am not aware of a crossfed switch in the B747. There is a realism setting for auto fuel transfer if this is checked than it should transfer fuel out of the center tank to the wings evenly in order to keep the propper balance. I know that in the real world a pilot has the abality to control which tank runs which engine and while on the ground they may want to run the apu from a different tank.
You would not want to run from one tank only the prefered method is to use the fuel from the wings while transfering from the center tank to the wings this way you empty the ceneter tank first.