my mother got visa at Beijing

This is good news, my mother just got visa in Beijing on 25/2.
I have questions:
(1 )The visa is only 4 months staying in USA, and single entrance. May I apply for her visa extension after she arrives?
she is 60 years old, she won't visit us after this time, so I don't mind if the extension causes her future visa refused.
(2) If I apply for extension, where to apply and when to apply?

Thanks very much for any reply.