Open ..steam/steamapps/workshop/content/410900/ and locate the folder containing the map you want to edit. The folder names correspond with Workshop ID's. Then copy it, and place the copy in steam/steamapps/common/Forts/data/maps/. You will also want to remove the publishfieldid.lua, and you may need to edit the displayname and map lua before it will show up in the Map Editor menu.

If you do decide to copy another player's map, please be respectful towards their work.