User Vivek Gite of nixCraft shows us how to add an encrypted ZFS pool on your FreeBSD server. He’ll show you how to do it with gpart and geli encryption system. You may view the full set of instructions at the link below.

I recently added hard disk to my FreeBSD based server. How do I configure an encrypted ZFS pool to store data on this disk? How can I add encrypted ZFS pool on FreeBSD 11.x server?

Introduction: ZFS is a file system for FreeBSD operating system. It is file system and logical volume manager originally designed by Sun Microsystems. A VDEV is nothing but a collection of a physical disk (such as /dev/vtbd2) file image, or ZFS software raid device, hot spare for ZFS raid. A zpool is nothing but a storage made of VDEVS (a collect of VDEVS). You can combine two or more physical disks or files or combination of both. This page shows how to create an encrypted ZFS pool on FreeBSD server when added a second hard disk to the server.

Full tutorial: https://www.cyberciti.biz/faq/how-to-add-encrypted-zfs-pool-on-freebsd-server/