However, I'm sorry to say that the amount of code it would take to incorporate this is so excessive that I don't think we will ever do it.
It sounds weird to hear that it would be a difficult thing, right? But there is no concept of a mouth slot currently. There are no commands for interacting with just your mouth, or indeed for any specific part of your body-- outside of wearing clothing and holding things in your hands.
In the end, you can post holding it in your mouth. Hell, if you have a cigarette on you and you want to just RP lighting it and putting it in our mouth and updating your look place to show it's in your mouth and then posing that you're puffing on it: go ahead. As long as you then trash the cigarette after your RP, that's fine. It's RP. You don't get anything from actually smoking a butt ICly.