Poor Man's Box 2
If you are good at soldering, or know someone who is, you could make up a cable to achieve the effect. You will need a suitable connector for the Icom (3.5mm mono jack plug I think) and a standard headset plug (0.25 inch). With a suitable cable, the ground is connected between the two plugs. The signal is connected via a 0.1 microfarad capacitor. This passes the audio, but keeps any dc levels apart.
You can then use a Y Adapter to connect your headset and the signal from the Icom in to the existing system. You could also plug our made up cable in to the P2 headset socket, assuming you are on your own. I’ve used this system to make a cheap ‘Box 2’ with a scanner for monitoring Volmet and such.
Good luck.
Richard W.