Here is a list of software which support mpc including some Mac OS programs. Maybe those programs can help you convert. Or you can switch to one of those players (instead of the extremely limited itunes)
If you just want to play MPC files, you don't need to convert them, just download Faasoft Audio Converter for Mac to play them. It's free. If you insist to convert them to MP3, Faasoft also can do this job, but it's not free.