you can add a poll to an existing thread just fine.* but you can't have more than one poll in a thread at a time (maybe this limitation could be removed, but the ui might be a bit confusing), and also you can't close a poll except by deleting it entirely (which is a more serious limitation that i intend to do something about soon)
*but only the creator of the thread (or someone with p_manage_threads permission, i think) can add or modify polls