I doubt FreeBSD can read btrfs volumes directly; however if they let you set up a fully virtualized VM (i.e. appears like its own hardware and you could install

I doubt FreeBSD can read btrfs volumes directly; however if they let you set up a fully virtualized VM (i.e. appears like its own hardware and you could install Windows under it), then you could create the correct sized VM, install your OS, then migrate.