Gamecube: Not possible by any means supported at this forum.
Older consoles: I am assuming you are talking here about games unavailable on the Virtual Console (VC), or games that you already legally own and do not wish to repurchase. There are some "VC injects" (basically people take VC games, swap the ROM and edit the banner, and reinstall). Most of them are illegal (i.e. unless you purchase the original VC game yourself, extract it from your own Wii, do the modifications yourself and reinstall it, it's illegal), and generally speaking you will get better results by using a homebrew emulator. The emulators built in to VC releases often have game specific optimizations and fixes, meaning compatibility is highly dependant on the base VC game, whereas the homebrew emulators should play the majority of games without issues. You won't get channels for specific games on the Wii menu, but you can install a channel for each emulator, which when loaded will display a list of your ROMs for that console. (There is also at least one multi-system emulator which would give you one channel from which you could load games for a number of different systems)