No Title of Nobility shall be granted by the United States: And no Person holding any Office of Profit or Trust under them, shall, without the Consent of the Congress, accept of any present, Emolument, Office, or Title, of any kind whatever, from any King, Prince or foreign State.
So a citizen can, but can then no longer hold any sort of office, unless approved prior to acceptance by Congress. Congress generally only approves of honorary titles.