Do you have the map with the unit on it or somehow download the unit only? If you downloaded the map with the unit on it go into the Object Editor into the units section, find the unit you want to move to another map in the units list in the space to the left, right click it, click copy, now go to your map and go to the Object Editor in the units section to the left, right click and click paste. This will also work with abilities, buffs, doodads etc.