Answer I
Open the editor, and load the map you want your mission to take place. Then press "Merge" button on the right side of the screen, and select the mission you had created on another map.
ThereÂ's another way: Open the mission in the old map, and select EVERYTHING. Press Ctrl + C. Open the new map and press Ctrl + V.
Both ways, youÂ'll have to copy all the stuff in your old mission folder (scripts, description...) to the new one.