NOTE: THIS WILL NOT WORK FOR CAMOS OR ATTACHMENTS EARNED IN THE BLACK MARKET.
This very simple tutorial will cover how to edit your weapon kits for use in any custom map.
First, go the the main menu, and Play Offline.
Next, go to Weapon kits in Zombiemode, and change your guns to whatever the hell you want.
Exit the game.
Now, go to C:\YOUR\STEAM\DIRECTORY\Steam\steamapps\common\Call of Duty Black Ops III\players
Look for two filed called "loadouts_zm_offline_0.cgp" and "stats_zm_offline_0.cgp"
Copy those files (CTRL+C), and paste them to C:\YOUR\STEAM\DIRECTORY\Steam\steamapps\common\Call of Duty Black Ops III\players\mods\usermap
and replace existing files.
Now when you load your custom map, your guns should have attachments ingame!