Everything I've read and seen so far indicate that's not possible in the current version of TvOS. I don't really see that until we have much larger Storage options either considering the current approach even impacts the 64GB which is the limits imposed upon developers regarding how much data can be stored on the device and how additional downloaded content can be erased at any time given Storage restraints.
There is a post in this very forum where someone downloaded a movie from their iMac to their ATV4 (they started movie and paused it and let it download entire video) then switched to a different video and back to the one they waited for it to download completely and it started downloading again.
That to me is the best indication of how storage will be handled AT LEAST until TvOS2 which hopefully lands next fall (beta hopefully in June).