Note: As of July 16, 2021, anonymous question asking is no longer supported. Please see https://qr.ae/pGMzdD for further details.
You cannot add or remove anonymity after a question or answer is added to Quora. More clearly, if you write an answer publicly under your name, you cannot choose to go anonymous later.
If you want to do this, you will need to delete the answer and re-post it anonymously. The same concept applies if you want to remove anonymity from an question you added anonymously.