what you need to do, is open a project, that project is your persistant level, you can then add existing maps to the persistant level in the levels menu. you can switch levels in and out on the fly either through a menu system or by streaming them in via some kind of trigger. every map in your game should be in your persistant level, just remember ( cause i forget all the time ) that you need to make whichever level your editing the current level, otherwise things get tricky, it’s a good idea to hide levels your not working on so you don’t get confused why there’s a garbage bag in your space ship environment :).