The process is aligned with the Mixamo rig. The provided parts are limited to the elements available in the template.
Any modifications would be manual, as far as I understand the given options.
In short, the needed joints need to be created, constrained to the given Rig. After the Skin Object is disabled temporarily, the new joints need to be weighted, with a clear change of the Character Object-based weight (100% for each point). After “Set Bind Pose”, the Skin object needs to be activated, and all animation for the new Joints will be optional.
That this will need way more attention than the suggested workflow might be clear without extra information.
I think the most straightforward way would be to add a Pose Morph and use this for Facial Expression. Which leaves the Rig untouched and allows it to work independently from it. The given sliders will allow for easy access.
Cineversity has a detailed series of how to create your own templates.
This is a series created for Technical Directors, IMHO, and it certainly needs some patience and experience.
I hope that helps.