You can still nominate topics during and after the voting stage

[franc6]

Just navigate to /nomination, enter your new topic, and it will be added.

This probably shouldn't work for non-admin users.

If we allow the admin to moderate topics during the voting stage, this is less of a problem, since the admin could remove anything that shouldn't have been added.

It could be useful for admin users between the nomination & voting stages, during the voting stage, and after voting has finished (at the US conference, we've sometimes added an additional "BoF" and it might be useful to add it into the DB after the fact for historical purposes).