Hi Wangtian 777,
there are even more solutions for macro work with the Rolleiflex SL 35:
Rollei also made a retro-adapter for the 50mm lens. if you wnat to do more "stationary" work, you can get the Rollei bellows, but this is a rather bulky solution, which you don't want to carry around on hiking trips.
i am usually cheating here a little bit, using a Tamron 100mm macro lens with the Tamron Adapt-all adapter. This is a very good lens and I have adapters for Rollei, Leica, Nikon and Pentax. you can find them at the used camera dealers or at ebay.