No you can’t. This comment is wrong. The Wii U is actually closing its shop and you can’t add funds to an account anymore.
https://en-americas-support.nintendo.com/app/answers/detail/...
https://en-americas-support.nintendo.com/app/answers/detail/...
Oh wow, how did Nintendo manage to be this stupid?
Except the cryptography and security has been bypassed. So the consoles have a homebrew app that lets you download and install to console and pirate the game. All done on the console.