Which modelling program do you intend to use?
If Max or gmax:
Import your chosen drider
Delete all but the head
Import pme0_head001
Examine all the differences including the pivot (Do not move the elf head)
Reposition and scale the drider head to exactly overlay the elf head... Note the pivot locations and direction
(apply Reset Xforms when satisfied...)
Rename the drider head to an empty "elf heads" value (less than 50 but +1 from your highest Pc elf head), add a "g" to the end of the new name, delete the original elf head.
Link the new drider/elf head to the AuroraBase, rename the base to the exact number of the head
(aka pme0_head049 without the "g")
Collapse all your transformations and then apply an AuroraTrimesh modifier to the geometry only
Save your file and then export the new model with it's texture to a folder (.mdl +.tga)....
Place the new model (and the old texture) into your override for testing in game.
Re-read the Custom Content Guide v3 about creating a plt
My recipe is a bit abbreviated (It's late here) but it should do what you want. Your new head may need some tweaking to make it truly unique (make a separate texture plt) but it should work just like that..
Happy modding....