It cannot be done in blueprints (at leas not that i know of). However you can check the code of Unreal Tournament to see how its done there (you can get it from github). I would start checking out how they made the main menu, and see what variables they are getting from the level, if any.
You may want to expand the UWorldSettings Class like mentioned here. I don’t know how you will get the data from the map without opening it, but it has to be on the Level selection menu from the Unreal Tournament code.