The easiest, safest route is to buy the headset, cut the cable about 1 foot from the DS end and solder on jacks for both the mike and speaker. Then wire plugs onto the headset end. This way, you can hook it back up and have a usable headset, perhaps even use the headset.

Another way would be to install a jack on the headset itself that disconnects the headset mike and allow injection of an alternate signal.