I don't know how to add a poll or do I have permission to create a poll at all.
newbies don't need permission to create a poll. that being said, if you already published the thread you are now unable to edit to put a poll in the thread. if you want to create a poll thread you would have to create another thread and include the poll in it before publishing it. anyway, in the future if you plan to create a thread that has a poll in it just click "post new poll" which is next to "new topic"