I'm quite curious about this because Tobi mention that the final stage of Sharingan is Rinnegan.
How does Sharingan transform to Rinnegan? Another transplant or a hidden jutsu?
You need to have the Eternal Mangekyou Sharingan, not just the basic form of it, combine with senju DNA. Whether or not you can put the EMS in a senju is debatable BUT you can definitely put senju DNA into an Uchiha with the EMS as seen with Madara.