Importing in Mixamo a character rigged in Blender

part 1

• ok all Mixamo rigs are the same but for the hand i.e, 5 digits vs a claw or nothing
• you can add bones to a Mixamo rig with face being the most common so you can animate a smile now that Adobe ripped out the blend shape system but you can't upload your own bones if they do the same thing as Mixamo bones i.e, Mixamo will not replace bones

now if you want to share the model I may suggest which rig to use but at this point only you can answer that

part 2

Mixamo uses Fbx which they don't really understand and worse keep changing so that creates errors in reading the bone rigs between Mixamo and what ever 3rd party software you try to load your animation into, things like crossed arms, folded skin, and busted animations are all caused by Fbx being misread and the fact of the matter is Autodesk does this by design so the point of this addon i use is simple... don't use Fbx convertion in your workflow but Dae instead

to be clear, its not Fbx verion 1, 2 or 3 so much as auto-converting it from one to the other that tends to break things

i use this one (has tutorial youtube and good notes) but there are others

https://github.com/enziop/mixamo_converter

arms cross because Blender is reading the Mixamo Fbx wrong... or because the Mixamo Fbx actually is wrong

there are plug ins for Blender which add the standard 4 Mixamo bone rigs... imo its worth a few $ to save time but yes you can do it yourself if you like

p.s, arms folding [at the elbow] is also an Fbx read error