Hi. As the author, I can probably modify the converter so that it is less restrictive on bone names, but:
- Once you import the Mannequin-based animations, you can easily retarget them to any character inside UE4, with some control over the bone correspondence and also some minor pose adjustments.
- You cannot just upload "any model’ to Mixamo. The uploaded model not only needs to have the right number of spine/neck/arm/leg sections, but also a bunch of extra leaf bones (fingertips, head top and toe ends). Without those extra bones, Mixamo will not animate the fingertips, the head and the toes.
- So in order to cover “any model” I would have to provide a “pre-processor” for your character FBX, that adds the extra leaf bones (and renames/reparents the other bones according to the conventions). I do not really want to do that, though. But you are free to do it manually in the 3D tool of your choice.