A frame orginally built for fixed gear probably does not have either brake mounts, derailleur hangers or cable stops. Some fixies do have replaceable drop outs so that could be overcome. You could use clamp-on cable stops. If there is no drilling or mount for brakes, something would need to be brazed or welded onto the frame. If you go to this effort for the brakes, the same person could probably replace the dropouts (and then repaint everything). This can get pretty expensive to go to all this effort.