It depends on wither you want to have a live mirror or an offline backup.
For a live mirror just do 'zpool attach' if your system BIOS or if this is SPARC OBP can see that drive it will be able to automatically boot from it in the case of a failure of the current rpool disk.
For an offline backup you can create this using 'zpool attach' followed by 'zpool split' or you can do as the other suggestion and create a new be on an alternate pool, both of those will result in a bootable but older version of what you are running. There is also the possibility of using 'zfs send' 'zfs recv' but that won't give you a bootable disk, but you could use snapshots with send/recv from the original to a bootable one you created using beadm to build a second bootable pool.
It all depends on how automatic you want this and wither the backup is supposed to be an older copy of what you have currently running or identical to is.