A new discovery thanks to SOMYST in the Discord, it's possible to change NPC weapons in a level.
You need to open the map's "BDD" folder then decode and open up the "objetos" folder.
You can find each "ID" of every NPC (To my understanding, "generic" ids like Officers tend to be copy/pasted" and change their ARMAS like with the player.
Here's some Russian/German soldiers given the STG44. (and they seem to not have animations for it but it seems they can still shoot you, at least at close range.)
Curiously changing their "equipped" weapon doesn't change the actual weapon they drop upon death.