Movable Type does not include a feature to export/import templates. The Import feature within Movable Type includes information regarding entries and comments only. If you are wanting to backup your entire weblog (not just templates) in its current state, you can do so using the Backup feature (Tools > Backup).
http://www.movabletype.org/documentation/administrator/maintenance/backup-and-restore.html
If you are wanting to duplicate all aspects of the original weblog (including content), the best option will be to create a clone of the original weblog. You can find more information about this feature in the documentation here:
http://www.movabletype.org/documentation/administrator/managing-blogs/cloning-a-blog.html
If you wish to duplicate the templates only, you could delete the content after cloning the blog.