Is it possible to add to a pre-existing rig? I'd like to add a facial rig to either the Unreal Mannequin or the Mixamo rig. But when I set it up and parent the new bones to the mesh the rig stays put and the mesh scales up by 100. Is there something simple I'm missing or is this just not possible to do this?
To answer my own question...when selecting the parent instead of "With Empty Groups" or "With Automatic Weights", Set Parent To "Object (Keep Transform)". Then in Pose Mode under the Bone Properties (little green bone) at the top you can see what the new bones are called. When weight painting you'll just have to add your own vertex groups named what each of the new bones are called. Then do manual weight painting. Kinda tedious but it gets the job done :)