The lighting system renders on the draw_gui event (by default). PostProcessingFX can render on either the draw_gui or post draw event. I would make an object with a post draw event and put this in the event:
ppfx_id.Draw(application_surface, _pos[0], _pos[1], _pos[2]-_pos[0], _pos[3]-_pos[1], _view_width, _view_height);
That should draw the ppfx shaders after the lighting system effects are drawn.