Sorry I'm not understanding how the inside humidity of the car has anything to do with the key barrel seizing up? Squirt some silicone grease spray with a plastic straw into the lock and that should stop the barrel from freezing up.
If you have a problem with the actual door freezing, then i would suggest your door seals need some attention. If the seals are ripped or torn (due to freezing and then pulling the door open and ripping them), then this will make a poor seal and allow more moisture into the vehicle. Closing the air vents overnight can help reduce this.
Also applying some grease on the door weather seal itself and on the body where the seal touches the paintwork can help prevent it freezing. I can't see how a central locking module is going to help with any of these issues to be honest.
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