It should be possible to add a choice not specified by the poll author.
I think this pretty much invalidates the poll, in the general case -- just start another one if you want to add options. We could maybe let you "Create another similar poll..." but haven't seen other requests for this.
A similar thing came up for us recently: "vote for a project name or write-in your own!"
Write-ins "invalidate" polls only inasfar as you think of polling as super-serious. Maybe just disallow write-ins in "serious-business" mode?