TL;DR:
Make Hordes and reduce their movement speed to -200%
DISCLAIMER: It is impossible to add or remove normal settlements from the campaign map. The method below does not create traditionally controlled settlements, but rather semi-to-fully autonomous settlements that are not designed for player use.
Make a new Horde faction and place its army(ies) where you want new settlement(s). Crack open the faction_bundles_to_effects table and throw together a new faction trait bundle by copy-pasting the regular bundle for that culture. The distinguishing factor is that you add the line "movement_range_mod" to the new bundle and set it to -200. You have to attach this faction bundle to your new faction, but beyond that you are good to go. There may be some other things here and there that I am forgetting, but this is the core of it.
Things you can do beyond this:
Make them vassals of whoever owns their region and make events that give "post-battle options" for destroying or subjecting them, with different options depending on the race. For example, if an Empire general attacks and destroys an Empire settlement, give the conquerer an event that presents the options to either sack or occupy. The former gives a sum of money and leaves the settlement destroyed, the latter recreates the same faction at the same location but under your vassalage instead, effectively giving you a foothold in enemy territory. Beastmen could be just granted funds from sacking without the option to occupy. If one wishes, this event could also be made to remove all remaining action points from the victorious army, akin to capturing a regular settlement.
After defeating them once in battle, they will lack the movement speed to "retreat" and will be destroyed instantly. So, after they are destroyed and 20 turns pass, make them "petition to rebuild" to the owner of the region through an event, offering vassalage with it. The owner can either decline this event and they will return again in 20 turns, or you can accept the event and they will be recreated at their original location.
One can combo it up with the new custom battle maps in the campaign, placing siege or village maps at the same coordinates of the custom towns and cities.
Combine it with a general movement speed reduction mod to effectively "make the map bigger/deeper", adding more stuff to do in terms of raiding and harassing while giving more reason to take your armies out of settlements. This kind of thing greatly benefits the defenses and income of landed factions, and so more Beastmen, Orc, and Goblin Hordes can be made to populate the world. In addition, more "Hordes" can be made to counter these and act as protectors, such as Wood Elf and Dwarf Rangers, the Inquisition, Knightly Orders, Norscan Ragnarites, and Bretonnian Merry Men (Herrimault).
This is also a great opportunity to include some humans in Sylvania in a lore-realistic way, providing convenient targets that you can use to get more bodies but also risk losing income and diplomatic reputation. Also, the High Elven exclave of
Elftown (A.K.A. The Continental Exarchate of the High Kingdom of Ulthuan) is located within Marienburg, and so can be placed right next to it in-game as a vassal of the Phoenix King when the expansion arrives. The Wood Elves of Laurelorn Forest can also be included in the game through this method, as they are within the region currently owned by Nordland.
Hope y'all find some good use out of this!