For a lossless codec, I would recommend SheerVideo. It costs money, unfortunately there isn't anything free like HuffYUV on the Mac. I've edited both of my Story Of projects using the PhotoJPEG codec at full quality in FCP with excellent results. I would just google for a mac program that can extract your 5.1 file into separate files to use in FCP. I know one exists and have actually done it before, just can't remember right now. Good luck.