Adding subtypes to an Eidolon (PF)

[Rift_Wolf]

Are there any rules for adding elemental subtypes to eidolons? I've been thinking of introducing an old idea of mine to my next campaign: A clan would nail elemental spirits onto its warriors, reducing his or her lifespan but becoming immensely more powerful as a result. Looking at the Synthesist archetype, I think I could do something similar, only the fusion is permanent and there's some fluffy downsides (Reduced lifespan, constantly covered in plate armour forged from raw elemental energy, social stigma, etc.)

Any thoughts on balancing this? They'll mainly be NPCs, I doubt players will really want to go through with the ritual themselves.