No, a P.O. Box in the United States cannot be used as your voting residence address. When you register to vote and request an overseas absentee ballot, your local election official in the United States needs your last U.S. residence address in order to determine your precise voting district. Your voting residence address must be a physical street or rural route address, or a location that you describe in precise detail.
However, you can use a P.O. Box as your overseas mailing address to which your ballot will be sent.