I'm very interested in this aswell, especially the animations, then i can make use of entire mixamo library for games ( https://www.mixamo.com )
I think castle doesn't support fbx either yet.
I know this exists, and it might serve some purpose for you: https://github.com/libgdx/fbx-conv
three.js can also load fbx including animations, maybe it can serve to you as a converter too?
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