I use FreeNAS for my personal usage and my clients
I only have 8TB at home to stream the HD video/audio around the house
Where my client has 40TB raw and 32TB use-able space (2TB HDD was used)
Both systems can have more volumes, but not as ONLINE expand. Because:
1st - when you are doing ONLINE expand, the data is at mercy of expansion process it takes a LOOOOOOONNNNNG time, because there are TWO process:
a) increase the raid volume - that handles by raid card - it can be days
b) increase the partition - this handles by the system - it can be days
2nd - If you don't have the back-up, then there is a chance you will loose everything
I would not recommend you to expand the NAS volume but rather add NEW volume so you can see more than ONE network drives
Here is what I did.
http://www.tomshardware.com/forum/265641-32-40tb-server-performance-issue