You can convert down using the Video Convertor. Or you can use the UHD-to-Bluray copy option.
In either case, you aren't going to be lossless, but you will be able to get a very good 1080p output. "Lossless" or "loss of quality" aren't exactly the correct terms here, as you are literally taking away half of the resolution of the picture, so there's no way to be "lossless".
This might not be the best/easiest way, but if I were doing it, I'd use DVDFab Ripper to get the movie in 2160p, then use DVDFab Creator to create a Bluray with just the movie on it.