Copy paste is on the roadmap: https://trello.com/c/HJMhC0hV
I also have another thing planned that would also help in this situation. Right now the actors are pre-configured. But I want to allow users to change or add to this pre-configuration. So you could add the enemies you want to the library, and then just drag and drop them onto your level.
But unfortunately none of the above is available yet.