We do not recommend modifying the headset cable. Headset extension cords or headset "Y" cables are also not recommended. Extensions should be limited to no more than 15ft due to the possibility of capacity coupling between the microphone signal and the headset signal which would cause a loss of high frequency response or oscillation.

CAUTION: DO NOT connect microphone ground and earphone ground together at any point.

Clear-Com does not normally recommend that a user extend a CC headset cable. However if you have to extend the headset cable out from the mains or remote panel it must be less than 15 feet.  This is to minimize potential cross talk between the mic and headset lines due to the capacitance build up in the cabling.
The two pairs (mic and earpiece) need to be individually shielded, on the XLR-4 headset connector XLR pins 1 and 3 cannot be connected together. 
You can contact Mike Wilson at www.bigskyengineering.com  or call him at 858-484-2409 If you want to build one, please do not make it longer than15 feet.  You’ll need one male and one female 4pin XLR connectors.  Pins one and two should be on a separate shielded pair from pins 3 and 4.  Best to use two mic cables and “marry” them together by taping them together every 6” or so… or encase them in a piece of shrink tubing.

Clear-Com Headset Specifications (From MS-704 user manual)
Pin 1 --- Mic common

Pin 2 --- Mic hot

Pin 3 --- Headphone common

Pin 4 --- Headphone hot
Headset Microphone Input : Input Type Dynamic Input Impedance >= 1KΩ
Headset Output Load Impedance >= 8Ω Output Impedance <= 25Ω