I haven't seen the MZ yet, but it sounds like you need to use the keyboard in FULL mode - NOT split. Then the midi in would access whatever channel you tell it to. Does your "old" keyboard have the ability to send info on any ONE channel? If so, it should work as you would like.
Again - this is all speculation. I am very big on only giving advice where I am certain of the facts. These are educated guesses, based on past experience with other keyboards. I hope someone will jump in and set the record straight with a "possitive" solution for you.