Expanding Capacity via extra drives

Hi,

In a QNAP I am able to add more drives to an existing Array and then just expand the Array's capacity.

To confirm, is this not supported within FreeNAS/ZFS?

Asking as I started with a 4 Bay server, now have an 8 Bay one and would love to start using my remaining Bays and so far haven't been able to find a straight answer other than creating another pool and putting disks over there and what not...

Thanks,