- How would one create new flexers entirely?
- They are based on Paul Ekman's Facial Action Coding System(Wikipedia). Basically you can make every facial expression with flex(or Action Unit) combination. And that combination is stored in an expression file. If your model implements these flexes(by keyshape animations), you can share facial expressions with other HL2 models.
I'm not sure why you want to create entirely new flex set, anyway I think it's possible. See
facerules_xsi.qcidefine flexes and their controllers. - N-neko