I did this today manually. The good thing about it is that if you keep the file, you can use it to store every mixamo animation you want, and it will work with any humanoid rigify rig.
Just import the skeleton, and start snapping a rigify metarig to the imported skeleton. Generate the rig. CHECK THE RIG CAREFULLY to ensure everything is lined up and oriented correctly. Start constraining the controls (yes, this means you cannot control the rigify rig any longer, bare with me) of the rig to the Mixamo rig. (Works with any other rig for that matter).
Experiment with the constraints to see what does what. Mainly you’ll be needing ‘copy’ transform constraints, mostly ‘rotation’. Sometimes, you may want a scale constraint, even an IK constraint sometimes.
Once you’re happy with how much your rigify rig follows the mixamo one, start importing your fbx’s animations, and simply copy the action accross to the rigify rig. Bake the animation on the rig with visual transforms ticked. It will be all weird looking unless you automatically disable modifiers, but your actions should basically work if you set the constraints up right. This file can now be used to store hundreds of mixamo animations, and you can import the actions onto any compatible rigify rig, which of course will not need constraints for the baked animation to work, and be easily editable.