As of version 22.214.171.124 there is a functionality to unlock any map for editing use. Maps are only locked when it isn't owned by you, but it can be unlocked by adding a special file into that specific map folder.
Creating the .unlocker file
You can create this file via command-line interpreter:
Placing the .unlocker file
After you've created the .unlocker file you then place it in the map folder (The one that contains all map components such as the Level.dat file). If you don't know where it is, you can find it in a directory similar to this (Example directory):
C:\Program Files (x86)\Steam\steamapps\common\Unturned\Maps
Alternatively you can right-click the Unturned in your game library in Steam and select Properties, then Local Files and finally select 'Browse local files'.
The file placement should look like this: