They aren't supposed to. They use the "your body is the temple of the Holy Spirit" verse (along with a few other scriptures) to explain why they don't smoke, drink, tattoo themselves, things like that. Hope this helped
They can, it's really a matter of personal choice. However, to smoke to excess would be wrong of them, just as it would be wrong of any other person. Unless they have taken a vow of obedience, and the mother superior says no smoking, at which point, they will not smoke as a part of following their vows.