I have a brand new NRS drysuit that I purchased for whitewater kayaking a handful of years ago...the waterproof breathable laminate seems similar to that used in breathable diving drysuits and is a little bit thicker than the material Aqualung uses for their Aircore fabric suits.
I thought about putting an inflator vavle and exhaust valve on it until I it dawned on me that the whitewater drysuit is sized for a totally different activity. You don't typically wear much insulation under whitewater drysuit as your paddling generates more than enough body heat to stay warm plus you have the added layer of your PFD and spray-skirt.
Unless you have a Kokatat that is a few sizes too big you may find that it is not very comfortable to layer under the suit for cold water use.
Put on some insulating layers and try your kokatat suit on...if it fits, you can try to convert it and if it does not hold watertight at depth you can pull the valves off and patch the suit...the holes only need to be roughly an inch or so wide in diameter for the stems of the valves to fit through.
Good luck with this...I am interested in knowing how it works out if you do it.
-Z