i'm surprised no1 has mentioned customize mii....
anyways, @ the OP, it would be very easy to do (albeit not worth the effort, but that's debatable) and u do not need to unpack any wads but u will need to work with them. Here are some step by step instructions on how to do this
1) purchase a game from Wii Shop
2) dump the game to a wad using any ONE of the following methods
a) bluedump v3 (if u use the non-fakesigning version then you will only be able to reinstall the modified game/banner to your own Wii, if u use the fakesigning version then it can be installed on any Wii)
b) wad creator
c) make a bootmii nand dump, then use showmiiwads (on your PC) to extract the nand dump, then change the showmiiwads mode to showmiinand, then find your game, right click it and pack it into a wad
3) So now that you have the game you purchased from Wii Shop in Wad form, all you have to do is edit the banner. Download customize mii, and run it on your PC, then load the wad, edit it any way u like (music, images, title ID, etc., but this program cannot change the actual animation, that is much much more difficult). When your done, click create wad and save it to your sd card or fat32 usb hard drive.
4) on your wii, launch your preferred wad manager (ie. yawmm or mmm) then install your custom wad, it should overwrite the original you downloaded from Wii Shop and u should see your custom banner. Enjoy!
5) Steal underpants
6)???
7) Profit!
I cannot stress this enough, make sure u have brick protection in place before doing any of this. That means u should have bootmii, nand dump, and priiloader (if bootmii was installed as IOS only). Bad/untested custom banners or themes can brick you, so make sure to protect yourself and wrap it up dude!