A company in Japan named Cyber Gadget is releasing an adapter that allows the use of the DualShock 3/4 Controller on a Wii/Wii U. This device by connecting to the port on the Wii Remote. Other controllers that can be used with this adapter include Cyber Analog Turbo 3 (PS3), Classic USB Gamepad (PC USB controller), and iBuffalo Arcade Stick 13 II (joystick).
Cyber Gadget is scheduled to be released November 30th of this year on their amazon page. Head on over to Amazon.co.jp to see it for yourself. Keep in mind that while the Wii no longer receives updates, Nintendo still makes frequent updates to the Wii U’s firmware. That said, any update has a chance of breaking the compatibility of this device.