Currently the WorldBuilder is "external" in that you cannot go from an active game and open the WorldBuilder for that active game/map.
You get into the WorldBuilder through the "Additional Content" selection in the main menu, and once you are in WorldBuilder mode you select to load an existing map or create a new one. There is no option to load a saved game as a map into Worldbuilder. Everything being done with the map is "pre-game", as it was in Civ5 & Civ3. You create a map to play on, and then you select that premade map when you start a new game.