Managing WordPress menus is not that hard. You can create and edit new menus with a few clicks. If you would like to move your menus from one site to another or copy complex structures, things are a bit more complicated. WordPress Menus Management is a handy plugin that lets you duplicate and import menus.
This script lets you duplicate any menu with a click. Export / import functionality is also available. If you import a menu with link to posts or pages that do not exist, they are created automatically. Of course, it is always a better idea to import all your posts and pages beforehand to avoid any issues.
This plugin can prove useful when you are moving sites or duplicating existing ones. It is a commercial plugin.