Out of curiosity (since I don't have any closed questions), is it possible to accept answers on closed questions?
Yes, you can still accept it, and answers can still be voted on by everyone. Other users can also vote on your question.
Only locking prevents these actions.
Yes, this is possible (see @DavidPostill's answer) and moreover, it happens quite often. Here are over 2,500 cases where a Super User question was closed and an answer was accepted next day or even later.