If you have to do this then for God's sake don't go lashing up something that will be plugged into the aircraft's intercom system, that would be illegal, irresponsible and potentially hazardous for the intercom system, ie a serious risk to safety. Just don't do it! See post above re isolation of voltages etc, this isn't something anyone, electronics amateur or otherwise should even consider.
Just stick the mic inside the spare headset and wrap it up in a coat or something, or put the heasdset around the recorder if the mic is built in. Make sure the heasdset mike remains unplugged.
It is also illegal to record ATC in UK, so as a Professional pilot you wouldn't do that either, would you?