Well lets start out with NAS devices are not officially supported as storage nodes. The only supported storage nodes are FOG servers configured as a storage node.
With that said, you can turn most nas devices into storage nodes as long as they can support nfs shares, a ftp server, and a tftp server. All three services must be able to point to the same storage location. I know I’ve personally configured a synology nas as a storage node.
Secondly yes you can configure a storage node in a different storage group than your master fog server. No replication will happen, but you can direct your image files to that storage group. It is much less complex to just add the space to your FOG server than doing it this way, but in theory it should work. Just be aware you can only capture to a master node in each storage group. You can not capture to storage nodes in the storage group.