I think, adding just one activity of smoking cigars and one activity for snuff will be simple, good even for current update and add a lot of modding potential. Cigars may be added as special tools made of leaf in craftdwarf workshop. Snuff will be just POWDER_MISC. Activity of consuming this drugs will be coded like eating or drinking alcohol but with small priority, like socialize. For effects this drugs will use INHALED syndromes, cigars will make smoke, but powder drugs will not.
I'm not sure about putting it on the same level as important functions for living life, like eating, drinking, and socializing. There's plenty of people IRL who have never done drugs and will never do drugs and are perfectly happy with that, myself included - it'd be more than a little odd for it to be a need for the notoriously hidebound dwarves.
Rather, I think there should be some kind of proclivity in the dwarf's genetics, one for how often they would like to go do a drug, the other for how prone they are to addiction (probably a modifier for a base amount statted in the drug). Only when addicted would it become a need the dwarf had to fulfill.
That way, there would be some dwarves who never do drugs, dwarves who only do drugs rarely, dwarves who try it once and get addicted, and so on.