We would like to use Humanoid, but in the year I have used unity I have been unable to find a way to have prop joints. I can’t imagine there is no way to do this, I have worked in AAA Games for 25 years and never seen a full body IK system that doesn’t allow you to have sub-rigs so it doesn’t strip animation off extra joints.
The only solution I have seen is to run 2 animators or layer with a blendmask runnnig the same animation imported 2x for the extra joints. This was a crazy early requirement we had for Audodesk HumanIK when we had it in our engine in 2006 at EA. I am rather astonished you can’t just say “yeah don’t remove this joint” and if it isn’t there in source it would just use the position/orientation from the imported skeleton. Or like HIK create rig extensions that can be shared.