Such a device doesn't exist as far as I know. The problem is that the NAS itself is a PC with built-in OS which accesses the disks which are normally setup in a RAID array which only the OS of the NAS knows how to access. It then shares the data to the network like any other server.
USB ports on a NAS are therefore sometimes only for sharing printers on the LAN, or to attach another HD or tape drive etc., so you can backup the NAS.
