This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Paul's post notwithstanding, the answer is "you can't, unless class B helps you out." Java is deliberately designed to make what you describe impossible, unless class B provides a "backdoor" so that A's "doIt" can be called independently of the override.