I've looked up something I've neglected previously - engineer/sapper units. There existed "Company"-size units as well. Obviously, they varied a lot in size (though always smaller than rifle companies), but I'd say in game terms they could be represented as:
Partisan Sapper Co - available 1/42-12/44
HQ - 1x Partisan Sapper (UNIT 044)
Platoon 1 - Partisan Sapper (FORMATION 106)
Platoon 2 - Partisan Sapper (FORMATION 106)
Engineer Co - available 1/45-12/46
HQ - 1x Engineer Sqd (UNIT 052)
Platoon 1 - Engineer Pl (FORMATION 105)
Platoon 2 - Engineer Pl (FORMATION 105)
This is from Anić/Gutović/Joksimović:
Narodnooslobodilačka vojska Jugoslavije (People's Liberation Army of Yugoslavia).
The other thing that could perhaps interest you is that the Ustashe had a number (5-8) of improvised armoured transporters. If you feel they're worth adding, they'd have the following stats:
Date: 5/42-10/43
Speed: 18
Crew: 3
Carry: 12 men
Weapon: Schwarzlose HMG (turret)
Armour: 1 all round
Survivability: 2 (fairly ramshackle)
This is what one of them looked like:
https://i.pinimg.com/originals/e8/73...ae85f74b3e.jpg
They would also need a new Formation. As far as I can tell, they were used as single support vehicles, no sections or similar.