Here Noesis
http://oasis.xentax.com/index.php?content=downloads- all Quakes models file viwer-anim and convertor and exporter(mdl md2 md3...)
Export in blender first: 3DS or OBJ(recommnded) and open it in this program :-)
This is for Quake 3 model, for right, go to the pk3s(with zipps): models/player models/monsters - like. pickup you choise(head of human - up part of the alien) in Noesis and export there to 3ds let say.. and then scale the santa hat in blender witin head size, and again export it with the head model as new head - Snata_head with the naccery texture making (the curctly name like "head", copy the original and change).
I dont think its have a md3 importer plugin in new Blender.
and then you script this, or just name it like Santa_head.. This I dont really know, sorry.
Packed(PK3) it in models/player/orginal player you choise/right head name.md3
Texture in this folder like the orignal
In the console you can command to replace the head. you most do it for each one.
admins, modrators, You can change and fix this post if is worng somethere.