Btrfs is in the Linux kernel. FreenNAS is BSD, not Linux based. You can of course rsync the data from one system to the other over a network cable; and you can

Btrfs is in the Linux kernel. FreenNAS is BSD, not Linux based. You can of course rsync the data from one system to the other over a network cable; and you can also use -c,--checksum option to do checksum verification (super slow but if you care enough to use Btrfs and ZFS you care about data integrity presumably).