the mechanics of making a digital sensor work with a camera built for film are not impossible but have always been really kludgy in practice. The camera must "wake up" the sensor before the exposure can happen, and a film camera just doesn't have that circuit. There is nothing particularly special about 6x6 format IMO other than you never have to turn the camera to portrait orientation.
If you are dreaming of a digital 6x6 back to take advantage of how cheap 500CMs are since they are strictly hobbiest cameras for the most part these days - you will be disappointed. For a digital sensor you really want a body designed to use it. The Phase One XF is actually a really, really nice MF system, and if you figure out how to buy used, the body is less expensive than a used Leica, anyway. Swapping sensors via switching backs is a real bonus - I have a CCD back (Leaf Credo 80) and have rented CMOS backs(IQ3, IQ4) , and am looking at an achromatic back...but my bodies and glass remain the same. It's a pro system with pro prices...but absolute pro results as well.