the basic answer is, no. But you might find the "Q" (a gamecube/panisonic dvd player) a little intresting. You can look at it on this site. http://gear.ign.com/articles/356/356908p1.html
As far as I know, the only way to get a GC to play DVDs is, in fact, to buy a Panasonic Q. The drive in the gamecube itself isn't designed for full-sized DVDs, so even just getting a case mod won't help you out at all.