Heya. I know this is a very old topic, but I guess someone could still be looking for info.
I’m in a similar situation, where I want to be able to create additional anims for my MetaHuman which I cannot find anywhere else. I’m going to go the roundabout method. There’s a tool online that helps with Mixamo animation conversion. That tool comes with a rig (just the joints) which is supposedly a one to one to the mannequin. So, here: Terribilis Studio you can find the tool (it’s free and clean from what I can see). If you download it, you will find an fbx inside. Open it in Maya (or whatever you are using). And just build a control rig on top of it.
That’s what I’m going to do in the next couple of days, and will try to export an anim and see if it works for my MetaHuman. If I succeed, will post it here in case it’s useful for you or someone else.