Adding air bags: can that increase payload capacity?

I'm not aware of a state where the payload sticker means anything. Most of the time a 3/4 ton with properly rated bags or timbrens can carry a given load as well or better than the same model/year 3500. If the payload sticker is an issue for you, use a sharpie on it. What you really need to do is know what the law in your state says. Usually you can register your truck to carry or tow whatever you want.