If someone was so inclined they certainly could model
some new armour, the difficulty arises in how you are going to animate
that armour and get it in-game. Basically you are stuck using existing models. All you can do is tweak vertices - just alter their position, not add or delete.
If anyone is interested in taking a crack at the helmet, I've sourced some reference pics that might be of use and made up some 3 views: