How to backup NAS using Time Machine
I wouldn't use Time Machine for this, it is best at backing up your local Mac. The NAS you have has a built in backup function that can use a local USB drive or a network share (as well as other options). So if you want to backup the NAS to a drive mounted on your Mac just set the share up on the Mac then configure the NAS to backup to that share on a schedule. If you really want to use Time Machine you would have to mount the NAS or more likely specific directories on the NAS then include those in Time Machine on the Mac, This article gives good info on backing up several types of network shares to your Mac thru time machine: http://lifehacker.com/5685547/how-to-set-up-time-machine-to-back-up-to-a-networked-windows-or-linux-machine