I'm pretty sure there are adapters out there. I have two arcade style controllers built with ps2 controllers inside. I use adapters for my Dreamcast and an adapter for the ps2 to usb for my computer. The ps2 controller to usb adapter I bought from RadioShack. It works great. I just use XP to set the buttons for mame or other games in XP. The ps2 to Dreamcast adapter I picked up on ebay. Do a search on ebay for a ps2 to gamecube adapter.
Just wanna add this. If you want to build your own arcade style controllers. Browse the forums at
http://www.arcadecontrols.com Very good web page on building your own arcade controllers or info on finding parts or adapters.