I prepared this custom version of Blender for you to ease the process of adding root bones to assets downloaded from Mixamo.
This is a portable version of Blender 2.81 to which I added the mixamo converter plugin from enziop (available for free on Github) and of which I modified the source code a bit so that the parameters that I need are enabled by default.
If you want to learn how to use it, I've made a video tutorial available below 👇