Currently it's not possible to add a poll to an existing thread.
There is a workaround however.
Create your thread without the poll as normal.
When it comes time to add a poll, create a new thread with the poll data.
Then merge the old thread without the poll into the new thread with the poll - make sure the destination thread is the new one and do not leave a redirect.
Your original thread will now have a poll.
There is a workaround however.
Create your thread without the poll as normal.
When it comes time to add a poll, create a new thread with the poll data.
Then merge the old thread without the poll into the new thread with the poll - make sure the destination thread is the new one and do not leave a redirect.
Your original thread will now have a poll.