Maybe if your NTFS drive is less than half full, at least I assume this is a limitation of this project since it mentions keeping an original copy... Still, belief in god seems about right, or you have good backups. I had about 2.5 TB on a 3 TB NTFS drive I decided to move over to ZFS, just rsynced to a few various drives since I didn't have that much contiguous space elsewhere (building a NAS later...), learned I had a file whose name is too long for either ZFS or ext4 and had to rename it, and after making a zpool out of the drive I just rsynced everything back. Doing it in place would have saved hours.. but only hours, on something not high urgency that doesn't require babysitting.
The backup is a reflink copy as per readme - that means data blocks are shared with live filesystem and don't occupy extra space but there's probably quite a bit of metadata.
