I am relatively certain that “adding nicotine” is a myth, at least in the way people think. They definitely did experiment with using tobacco that has higher nicotine content and they also put additives in it to help it burn more quickly delivering the nicotine more quickly.

But overall, cigarette manufacturers would actually prefer you to get less nicotine than you are used to. “Light cigarettes” was absolutely genius for big tobacco. All they did was allow more air to enter the filters, so less smoke and nicotine entered the body. People smoking “Light cigarettes” assumed they were healthier, so they could smoke more of them… which was perfect because you needed to smoke more to get the same amount of nicotine. However all of that can be subverted simply by slightly wetting the outside of the filter, or by hooking your finger around most of it and blocking it.

Unfortunately because of the way lungs work, adding a little bit more air into the smoke mix has a negligible impact on harm to the lungs. So it just meant lungs ended up being exposed to smoke for a longer period of time, on average.