Poincaré invariance of the Maxwell equations in Minkowski space enables one to generate in a usual way new solutions (corresponding to new sources) from a given one by means of the pullback operation. A method is proposed generalizing this to a wider class of diffeomorphisms f:M → M of a Lorentzian manifold (M,g). It results in the simple explicit formulas for obtaining the new field Ff and its source jf from a given pair (F,j). Some applications, e.g., a uniformly rotating or radially pulsating source, are presented.