Yes, you can apply for an extension of tourist visa through the exit-entry administrative bureau of public security bureau once you are in China, you can apply for extension 2 days before the expiry date of tourist visa. it is impossible for yo to convert tourist visa into a work visa within Mainland China, you can go to Hong Kong to apply for work visa. You don't need to have proof of a return flight ticket to enter China, but the airline company maybe refuses you to board without the return flight ticket.