Any pump or tank could be used to feed the APU providing the crossfeed is open. The key point is to pull the circuit breaker for the Left Aft Boost pump especially if the left tank is empty as it can't be controlled from its own switch when the APU is running.
That's it in a nutshell.
It's possible, but why would you want to do it?! If you fed the APU from the Right main tank, you would only start an imbalance from that tank now, instead of the left!
Running fuel from the CTR tank would be the most logical solution if you were planning a protracted period of APU operation.
Lightning5
but how do you start the APu when no AC power is available.
Ummm...either I misunderstand your point/question, or I don't think you meant to ask that did you...?! Batteries...perhaps?!!
Cheers