You can find this tool under the Atoms menu: Atoms > Assets > Mixamo Character Variation Builder

Press on the "+" button to add a new agent type to the list, then press the browse button to add FBX files to the agent type entry.
Each FBX will be used a variation, the name of the variation will be the name of the file itself.

You will also have to specify a destination variation json.

This tool is currently available in Maya only and it runs by importing the FBX into the current scene and exporting the geo and material files from there.

This tool currently only export Arnold materials.

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