My short answer is: don't. This rig is designed for Mixamo, not to have any other rig, that has many more elements, been used with it. You might explore to set up several arms during the initial set up, but it wouldn't be a Mixamo compatible rig anymore.
The medium-length answer is: (Start at: 00:34:36)
I would suggest tuning in tomorrow, Monday, at 09:00 a.m. Pacific time and asking this exact question.
https://www.youtube.com/watch?v=j-VGxPgLE0c
( I QA and Timestamp the shows, so I'm super curious about the replies you get. Learning rigging should be based on as many sources as possible.)
The long reply is this link below:
To get an idea about the inner workings, there is so much in these templates that I would even start to explore to hack into its structure.
If you would like to change the template, that is explained in a more extended series and is a little bit old (interface.
https://www.cineversity.com/vidplaylist/creating_a_custom_template/
When you see how much instructional data is provided, I have no idea how to communicate this in a single thread, nor would I know how to communicate about the Python content. In other words, changing this template or structure is not my advice.
If you explore the rig, you will find that it is not one rig; it is pretty much three rigs, plus the eventually connected Mixamo from an external source.
This means the whole setup is based on four rigs; otherwise, why use the Mixamo if you don't connect it to the external rig?
You show me in the picture that you have placed just a few joints somewhere. Those will have minimal impact, as nothing previously used (external, IK, FK, nor bind) will provide them with individual information.
To call it a Mixamo rig at that point would be false.
My suggestion: make the rig edible (Character Icon active, press the C key) and place the extra joints where you like to have them.
The setup as IK/FK and Controllers can follow the same way as the default joints. Set the constraints to the IK/FK options are given as for the rest of the rig.
Weight the extra elements to the mesh, and perhaps smooth areas where previous and new weight needs to be defined.
With an Animation System, Animation Clips, that shows the same Hierarchy, it is easy to mix animation sequences.
All of that said, based on the information I got, while images have nearly no information as a project file would have, just for future questions. Thank you.
My best wishes for your project