If you subscribe to a mod on steam that has a map (either that's all it is or there's a map as part of the mod), once the mod downloads into your MODS folder, you can exit the game, start up Worldbuilder, and then instead of loading a map from the default directory worldbuilder uses you can browse to the CIV5/MODS/Name-of-the-mod folder and select the map file from there directly.
You can also just use your windows browser and copy the map from the mod folder to the folder Worldbuilder uses to store maps.
The default folder where Worldbuilder stores maps that you've created is My Games/Sid Meier's Civilization 5/Maps