Ntfs2btrfs is a tool which does in-place conversion of Microsoft's NTFS filesystem to the open-source filesystem Btrfs, much as btrfs-convert does for ext2. The original image is saved as a reflink copy at image/ntfs.img, and if you want to keep the conversion you can delete this to free up space.
Thanks to Eric Biggers, who successfully reverse-engineered Windows 10's "WOF compressed data", and whose code I've used here.
Compression support requires zlib, lzo, and/or zstd - again, they will be in your package manager. See also the cmake options WITH_ZLIB, WITH_LZO, and WITH_ZSTD, if you want to disable this.