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Do not modify the built-in PE pose banks — you will break stuff.
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Adding custom poses and dances can be done easily inside the Poses Extension Prefab:

Clicking on the prefab will give you various options, which are unique to each instance of the prefab and will be applied on-build.
Poses need to be provided the Poses Extension-way: A single animation clip, containing one pose per keyframe.

Each frame on the timeline represents a full pose.
If you want to have a smooth loop similar to the integrated poses, make sure that your first and last pose are the same.
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Useful Notes
Your pose bank animation clip should be set up like this for best results:

Your pose bank animation clip should match these import settings, else you may get weird offsets and wrong behavior when scrolling poses.
Just slot your dance animations into the three provided dance slots of the prefab’s modding tab, and you should be good to go.
Adding more dances is not supported and must be done manually by editing the Action Controller.
Dance animations should not need any special setup, just make sure “Loop Time” is enabled.