UPGRADATION OF HDD IN THE EXISTING NAS

You can switch to FlexRaid (if you aren't already using it), and then create a new volume on the NAS using the new drives.  If you are adding 20 drives, you might want to create more than one new volume.

You'd then need to create new shares on the new volume(s), and possibly shift some shares from the current volume to the new one(s).  Shifting can be done with with the built-in backup jobs.  All the home folders need to be on the original volume, so you do need to take that into account.  You will need to pay attention to the free space usage, and occasionally re-balance the storage across the volumes.

It might also be possible to expand the existing volume - we'd need more information on what RAID mode you are using now in order to tell that.  Though as the storage gets bigger, it makes sense to go with more than one volume.  If you do expand your existing volume, you should make sure your backup is up to date first.