Under POL 0.95 and converted 0.95 to 0.97 Distro death.src in \pol\scripts\misc is the script that fires when an NPC dies.
It's fairly easy to spawn a skeleton at the dead zombie's location.
Assuming you want a corpse of the zombie to remain on the ground and simply spawn a skeleton you can add the following code just before the
endprogram statement in death.src
Code:
if(GetObjProperty(corpse, "npctemplate") == "zombie")
CreateNPCFromTemplate("skeleton", corpse.x, corpse.y, corpse.z, 0, corpse.realm);
endif
I am pretty sure that will work fine. If you are running pre-0.96 POL you will have to remove the 'corpse.realm' parameter from the CreateNPCFromTemplate function call.