It's not this easy, is it?

It's that easy.

code/modules/mob/living/carbon/damage_procs.dm

@@ -120,6 +120,13 @@
 	adjustStaminaLoss(diff, updating, forced)
 
 
+/mob/living/carbon/applyFatnessDamage(amount)
+	if(!check_weight_prefs(FATTENING_TYPE_WEAPON)) //Typically, this kind of damage would be applied to people through weapons.
+		return FALSE
+
+	adjust_fatness((amount * FAT_DAMAGE_TO_FATNESS), FATTENING_TYPE_WEAPON)
+
+
 /** adjustOrganLoss
   * inputs: slot (organ slot, like ORGAN_SLOT_HEART), amount (damage to be done), and maximum (currently an arbitrarily large number, can be set so as to limit damage)
   * outputs: