You could fake it to some extent with a script .... as time passes, reset the skill level lower. AI units with lower skill run slower, think slower, shoot squinter .... all the things you associate with fatigue, thirst etc. You could also effectively reduce the amount he could carry, but constantly checking how much ammo he had and making him drop some if it was too much.
It a soldier goes to a fountain, or spends a sufficient amount of time without being in combat, you could make his skill rise again. I don't know how you could differentiate between different types of fatigue/stress though .... but one general thing would probably be sufficient.
This is all in theory of course, don't ask me to write the script. But the command is setSkill