Can multiple characters share a base rig but still use custom bones? I'm currently rigging a feline-humanoid character for a UE4 game project using the Rigify add-on. However, this character has some extra features than that of a standard humanoid, including a tail, whiskers, cat ears and hair that I would like to rig as well.



My goal is to animate the base Rigify humanoid to perform common actions (idle, run, etc.) that I can recycle for any humanoid character, but also somehow augment this rig to include this character's unique tail, whiskers, ears and hair to animate alongside the common shared animations. Does anybody know how one might achieve this?
After you generate a rigify rig, it becomes a standard armature, that can be edited adding new bones and parenting them to the appropriate bones. 

The only thing to consider is that the Rigify rig is quite complicated, it has a bone layer with all, normally hidden, deform bones, a couple of layers containing "service" and "machinary" bones, and a lot of layers which organize all control bones (thoose who have custom shapes). 

To avoid problems of interference with the structure of the rig, I think it's better not to move, rotate or scale any bone of the original rig, and use the deforming bones (those who contain "DEF" in their name) as parents of your added bones. You can find them in one of the last bone layers (normally hidden, you have to atcivate it in the bone layers tab of the armature properties panel).

 

If you perefer to have a separate rig (and animation) for your added bones you can use copy transform constraint (in world space). In example you create  a head bone in the exact same place of Rigify head bone, set a copy transform constraint with the rigify DEFhead bone as target, and use it as parent for your ears bones.