There is a lot going on with that option. I can't believe it would be worthwhile to try and add at a later date, that would be like trying to convert a gas to a diesel. Just order or accept the one without.
Can it be added? Older trucks it was very difficult and expensive, but it can be done. If you want to tear out the interior dash it might be something you want to do. Then you need to turn on the feature with ForScan or the dealer.
How well does it work?? Basically it's an electrical element that sits inside the climate control housing behind the dash, it works when the outside air temp drives it into the parameters. For those who have it they love it as it takes the bite out of a stone cold interior. Those who don't have it wish they did....(me included). Is it 100*F in 10 mins...no...But it does help get the warm air going until the ECT is up high enough to take over the task.
My experience has been that its not really that big a deal. I've tried it both ways. And it seems to start warming up in the cab just a couple of minutes before your normal heater kicks in. Ive talked to others that have it and it's the same thing. Just speaking of my experience and a few guys I know