If you want an additional Nintendo-branded Mario Kart Wii steering wheels, you won't have to buy another copy of Mario Kart Wii. Nintendo plans on selling extra steering wheels on April 11, the same day Mario Kart Wii debuts in Japan. Each extra steering wheel runs for 1200 yen ($11). Of course, Nintendo's Wii Wheel isn't required to play Mario Kart Wii. Ubisoft's wheel should work fine and so should a standalone controller.
Original source: Famitsu - Japanese