every single unit, gots its own nand backup
you can't download a nand backup of a wii from a friend and put in your wii, (just like people, everyone is unique; you dont want to give wii an allergy becuase of the wrong thing xD)
WII U is not hacked. you can make a nand backup (lets say, you want to keep a backup of the console at firmware 5.x) you need to open the console, and use wires and hardware...
i am not sure about what hardware, on a nintendo 3ds you just need to use a SD adapter. HOWEVER for wii u you might need an arduino or a rasperi pi, or other electronic "machines"
someone can tell you better what you will need
BUT KEEP IN MIND, THE ONLY WAY TO BACKUP YOUR NAND IS TO OPEN THE CONSOLE WITH SCREWDRIVERS AND THAT KIND OF STUFF. IF THE CONSOLE IS NEW AND DEPENDENT ON YOUR COUNTRY\NINTENDO RULES, YOU WILL LOSE THE WARRANTY