Fire/Frost/Shock and the respective resistances seem to be hard-coded?

Can I add new damage types with respective resistances?

I want to add cutting, blunt, and piercing damage types, and then make armor suits resistant to those by specific %'s.

IE cutting damage does slight drain health effect and is great against softer armor, but plate armors are nigh-invulnerable and swords do little damage to tin-can suits.
Blunt is negated by armor with more padding, but fairly equal damage across armor types. Does fatigue damage too.
Piercing gets no bonuses against lighter opponents, but gets the ignore-%-armor like vanilla-blunt weapons can, making them great for opening tin-can suits.

Every piece of armor would display damage type resistances like enchanted frost/fire/shock resistances display.

Not sure what parts of all this are possible.

Not that this matters, and not that I'm an expert (I'm quite the opposite, actually) but wouldn't Chainmail counter slashing/swords/piercing, and plate counter blunt? I believe that's what they were designed for, and arrows and swords can actually go straight through plate mail. I donno...

I wanna say if they are hard coded then you are out of luck, and I would assume, since those have mainly to do with magic, that they are. A lot of magic seems to be hardcoded so it would be no surprise if the resistances and damage types involved were as well.

Pretty much. Swords will still do blunt damage through chain. Chain and scale (Why isn't there more scale armor than Dragon scale?) would be top-tier light armor. Plate doesn't completely counter blunt. Plate deforms and transfers shock to the wearer pretty well. Slashing metal plate does almost nothing as opposed to a flail denting chest plates.

I guess adding back in medium armor (light plate over chain, light plate and scale, banded mail, etc) skill-trees as well is hopeless if I can't add more depth to armor-types?