Copy a theme with its stories from one backlog to another

I want to be able to build up a library of stories that can be re-used in new backlogs.  So I would like to be able to cherry pick stories and themes (complete with their stories) from project backlogs into a library.  Having then established a library I would like to be able to cherry pick themes and stories into new project backlog so that I don't have to re-write them.