not yet its not moddable ... not to play copied games anyways.
you can mod it to make it region free, making it possible to play JAP games on USA gamecube. I did this when I had my JAP gamecube so I could play Rogue Squadron 2 since it was released about a week before USA gamecube was released. You can either bridge the 2 points in the cube that change the region (to make it FULLY USA so you can't switch back to JAP), or you can connect the 2 points to a $2 switch you can buy at radio shack, which I did. You could switch between USA and JAP mode by pressing the button when its turned off.
You can play backups however on the cube, but it's a pain in the a$$ and about %95 of the games play back with some problems (slow graphics, music messed up, other wierd stuff). The reason is because it streams the game from the PC. You hook up the GC to the PC via the broad band adapter. You then load up this GC Loader on your PC. You then must use an original copy of Phantasy Star Online in your gamecube and go to the Online part of it, and this is where you use the loader on your PC. You load up a GC iso on your PC and then it streams it to your GC somehow. I have not done this myself as I sold my gamecube a while ago because I dont like it. They recently released a tool that lets you rip your own GC images from isos via your DVD-rom. You can also download them off the net if you know where to look ...
Again I haven't done this myself yet, so some of my steps may be wrong. But thats the whole gist of the idea that you need a BBA, PSO, and the loader on your PC.