It will fit if it has the 2 stay pockets on the back side and you have a k-connector to mate the 1” strap to the 3/4” webbing under the KU shoulder harness strap connecting area.
Newer model belts don’t have the stay pockets any more. Duplex belts also use 1” straps vs the 3/4” strap of the KU Belts.
Make sure you get a pic of the back side of the belt before buying it. But yes, regular duplex belts can work on the KU packs with a few caveats.
The stays will definitely fit in that large slot pocket but I’m not sure what the long term effects will be or how solid it will feel with any type of load. I’d be afraid the stays would rub a hole in the pocket. Pretty sure that’s why they used individual stay pockets made of hypalon to add protection and provide a more firm hold to the belt.