a close up lens has a positive number of dioptries and reduces the focal length of the camera lens. A tele-lens with a negative number of dioptries in front of the taking lens would increase the focal length and than the built in camera lens is too close to the film. You would need more distance between lens and film to get an infinity focus which is not possible with a Rolleiflex. Such "Distar" lenses existed for bellows cameras which have enough distance. The so called tele converters do the same but are mounted behind the taking lens, which is not possible with a Rolleiflex too. The Mutars are very complex special lens systems (large, heavy, expensive, rare).