You will find that a regular convertor from Radio Shack may, but most likely, may not work with the intercomm as the impedance values are different. I assume your goal is to record the cockpit conversation. You may be able to search on google and find a schematic on modifying radio shack parts to match the impedance.
Alternatively, if you have one of those David Clark-like headsets with a smooth cup, you can buy a telephone suction mic from Radio shack and just attack it to your headphone. It works pretty well and is very cheap. (under $5)
Jeff