Mannequin files are skin files stored in the ui\models\mannequin folder. Those are so far the only ones that work, check the Knowledge Base to find out why, I forgot. The thing is you'll have to re-skin one of those mannequins to get your custom ones, and that's the hardest part: you have a very limited base skins amount and will most probably end with repeated poses, so it's pretty tricky to balance things out, but not impossible.
You'll have to learn to re-skin, using TextureFinder and MUA Skinner. For Havok, you can take parts of his skins and align them properly with the mannequin skin when possible, that way you'll get pretty close looks. You are welcome to try and tell us if you get something good.