You can create a model from scratch, but you have to rig it to an existing skeleton. I assume that the skeleton itself is hardcoded, with the number and names of the bones must be what we have already. BUT, there's a skeleton with an extra bone: fs_slinger_new, where there's an extra bone called Bone_weapon or something like that. If you use this skeleton as a base for your model (aka. the Bone is present when you export it) and the skeleton file you use in the game is NOT based on this, then the model would get messed up bad in-game because the skin modifier would get confused by the extra bone. The reverse also produce the same bad effect. So I'm assuming that the bone names in your model do get exported into the .CAS file even if there's nothing rigged to it since the presence or absence of a bone do cause problems.

So, as of now, we can't add or remove bones from the pre-existing skeletons. We also can't change the xyz coordinates of the bones (EXCEPT for Scene Root), meaning only the rotations are read. However, as I've said above, there exists a skeleton with the extra bone so it might be possible to create new skeletons with extra bones but that seems to be hidden in the skeleton file or the packed files. There are at least some hidden files in there. The .evt files, for instance, are hidden and cannot be extracted by Vercingetorix's tool.