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 a .unlocker file just by renaming anything to '.unlocker.'. For example, a text document (.txt) or even an executable (.exe).
Make sure that you have "File name extensions" enabled in your Windows Explorer, otherwise you might create .unlocker.txt (or other file extensions it was originally) instead of .unlocker file, which is not the same thing at all.
Also make sure to include the fullstop at the end, or else Windows Explorer tells you that "You must type a file name". You can alternatively 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: