If you want to add a map to lava survival list of maps. First you have to make sure that the map you want to add is located in lava/maps directory, then you have to just add the map entry to maps.txt. It will be loaded automatically when needed.
If you want to keep maps for freebuilding you should change game-mode to freebuild or lava/freebuild and then maps will be loaded from the list in autoload.txt file.