I keep telling my fiance to do this, but he won't.
It can be done, but it takes a long time. I don't remember all the technical jargon, but basically what's happening is that by keeping the skin stretched taut all the time, the cells start dividing up and eventually create additional skin. So then you have more to pull taut. And you keep doing this until you eventually have a foreskin. Though it won't be a true foreskin, obviously; it's lacking all the nerves and whatnot that were in your original foreskin, but it will still serve the purpose of protecting the head (so you'll lose the callous caused by constantly rubbing on clothes, and gain sensitivity because of it) and will have all the benefits of a foreskin for any sex partners you may have. There's plenty of info online to learn how to do it if you want to go that route.