Migration from Freenas all ZFS volumes to another machines lvm is pretty simple.
create on you new machine logical volume and start netcat which will write everything it receives to the new LVM of your choice:
# Source box
dd bs=16M if=/dev/disk/by-path/iscsi-mysql-lun-0 | nc 192.168.1.254 19000
# Destination box (192.168.1.254)
nc -l -p 19000|dd bs=16M of=/dev/vg0/mysql-data