How to add additional disk (da1) in your zroot:
gpart create -s gpt da1
gpart add -s 512M -t freebsd-boot -l gptboot1 da1
gpart add -s 2G -t freebsd-swap -l swap1 da1
gpart add -t freebsd-zfs -l zfs1 da1
gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 da1
zpool add zroot gpt/zfs1