The Chiss addon is just a skin. I'm not sure how many different head variations this addon has, but I myself am planning to release a Chiss pack with 8 different head variations(minor variations though
). I'm still working from the demo here, but when i finally get the game I'll put it out after it's been tested
. It's easy to add new skin files, but if you actually want them to show up in game, you'll have to make an icon as well.
For example, I'm assuming you've had some skinning experience: Say you want to add an extra human head skin. Just look at the existing head skins. There named a1, a2, b1, b2 and c1. Depending on the surfaces you want to use for the head(check out the different head surfaces in modview), you can just add a skin file called for example, head_a3.skin or head_b3.skin or even head_d1.skin. As long as you have a head icon to go with it and the skin has been properly made(read: works in modview)it should work in game.
Now if you want to make completely new head models
, that's a whole other issue and you should ask that in the modeling thread.