It looks like the 3200 supports the newer XRAID-2, so you wouldn't need to swap out all the drives for larger ones, just a few to gain new space.

As for snapshots, thats generally the term applied to creating a new share that represents a normal share, but frozen in time. New changes to the share don't show up in the snapshot. Typically this is used for allowing backups to occur to stable data, or as a basic revision control setup. Space for allowing snapshots to be turned on would be carved out of the RAID disk space, and wouldn't be reliant on an external storage box.