Warm air rises, whether it's 'moist' or not.
If you run just the basement/main-floor furnace, and not the second-floor, does it keep the second floor warm in the winter? If not (and I'm sure the answer is 'no'...), likewise a humidifier on the lower-levels furnace isn't going to magically humidify your second floor bedrooms.
Most household humidifiers are of the "bypass" types, and any furnace installer could install one for you. These are attached to the side of the supply duct and either have a round, spongy pad on a drum that slowly rotates into a puddle of water and passes the moist pad across an opening into the supply duct, or a metal pad that air just flows through. The moist air is pushed into the supply duct by a "bypass" duct that is hooked up to the furnace supply further upstream. These humidifiers work, but are generally very inefficient and mostly suck ass.
What some manufacturers have started offering are steam humidifiers, which look like little miniature versions of the humidifiers used in big buildings. These have a small boiling vessel within which water is boiled to steam, and the steam is directed into a dispersion tube stuck in the supply duct downstream of the furnace. These are the cat's meow by comparison to the bypass types, but I have absolutely no idea how much installers want for them. Definitely more. Maybe you can be a guinea pig for CP and shop around.
