Well, it could be possible if you're willing to shell out some Wii Points for the add-ons and stuff.
Or if someone MAKES addons (aka. similar to WoW addons)
And you download those, stuff 'em on an SD card, and slap it in your Wii, and the author somehow makes it readable to the wii, and 'add-able',
Then yes, it could be possible.