As far as I know, there is no specific Warhammer fluff on this. However, the Empire is based on the Holy Roman Empire around 1500, and the (structure of the) Church of Sigmar on the Catholic Church. As such, a Warrior Priest named Wagner would be normally addressed as Father or Reverend in personal conversation. If you would be talking about a Priest of Sigmar Wagner, you might refer to him as Father/Reverend Wagner, as (Father/Reverend) Wagner, Priest of Sigmar, or, indeed, add anything else of distinction, in particular noble descent, e.g. Father Wagner of Bogenhafen, or Wagner of Bogenhafen, Priest of Sigmar etc.