"Necropolis" - changes to creatures of levels 6-7.

Is something like that even possible?

It is possible. Try changing the resources so that, for example, shamans receive bonuses of +3 to stats and morale instead of +1 when performing a sacrifice. The game will completely ignore this, because +1 is hardcoded into the executable, not the resources. Or, the duration of berserk, no matter how you change the spell in the resources, it will still remain 1/1/1/2 turns, because that's what's specified in the executable. Or, the destructive beam in a magic attack, no matter how you change the spell in the resources, it will still reduce 3 defense (regardless of the base constant) + the modifier for the skill.

In other words, the data in the resources can easily be ignored by the executable, i.e., the game itself.