Correct me if i'm wrong, but I don't beleive so. Checking so myself by going to edit my own thread, I don't think your able to add a poll to an existing thread. Tho, i'm not entirely sure.
~ Whisper
Actually I believe if you scroll to the bottom of of your thread, there will be a few options on the right-hand side, one of which will say "add poll." There you can add options, etc. I'm not entirely sure it'll work (although I can't see why it wouldn't) since I've never made polls before, however it's worth a shot.