Hi John, I was looking into this a year ago. If you search the net you
can find schematics for a 4 person aircraft intercom with aux in.
However, I could not find PCB software able to deal with all the chips
and routing, and flying wire connections are not really a great idea in
a shaky cockpit. Parts for a mono version cost around $60 plus PCB
printing, etc.
If you have the software or patience to build your own board then it's
an option, otherwise you might want to consider what RST or Sporty has
to offer.