For starters, I recommend having Notepad++ [notepad-plus-plus.org], it makes work in .asset/.dat files so much easier.
You should also know where you Unturned folder is located. It should be located at "C:\Program Files (x86)\Steam\steamapps\common\Unturned"
You will also need a Map with Bundles. To do this all you need to do is make a Folder in your map's files called "Bundles". This allows you to bundle things with your map, rather than making them an Required Items in the workshop's Download.
Inside the Bundles folder create a folder called "Assets" and inside that create one called "Zombie_Difficulty".

Now, it's time to make you .Asset file. This file will contain the Info that the Difficulty GUID uses. You can either make your own from scratch or get a preexisting one and edit that. You can find one used on Germany in the games files "Unturned\Bundles\Assets\Zombie_Difficulty".
Copy any one of the .asset files and paste them into the Zombie_Difficulty you made earlier. Now, it's time to give it a new name, name it something like "MyMap_Town"
Now, open up the File, and you should see something that looks like this
Now, we need to get a custom GUID. Thankfully, there is a website that can generate one for us, guidgenerator[guidgenerator.com]. Now generate 1 GUID, and uncheck all boxes and click Generate. You should now get a unique GUID (There is no guarantee that it will be unique, though it's very, very unlikely it will be used)
Now, copy this GUID and paste it into the file. Now, you can mess around with the spawn chances of Zombies, the maximum value you can put in is 1, which causes there to be a 100% chance of that type of Zombie spawning.

This step is really easy, take the GUID that's in the file, and Paste it into the "Difficulty GUID" box, then bake the Navmesh. If all goes well, this Navmesh should now only be spawning Zombies of your specification. If it's not working as intended, Close and Reopen Unturned.

Special thanks to Th3o, who helped me figure these out.