Do you mean some third party NAS device attached to your network?
The answer is - maybe.
Some work, some don't. They need to support the Time Machine protocols in order to be recognized over the network. You'd need to check the specs for the specific NAS device you're using to see whether it's an option or not.
We have a Drobo SAN B800i that is the target of all our backups. You use Drobo's management software to create iSCSI LUNs that are HFS+ and mount on the server as if they are internal storage.
You just set the LUN as your backup volume.
One caveat: iSCSI performace degrades noticablity over slow (10/100) ethernet.