Need help?

[The End]

Some people added a few buttons (like Foot Curve, Penis Bend Tip and others) long ago.

1. Is it possible to add a few more? Like "spine_joint03" or "spine_joint03" direct control so you can tweek the spine length on the timeline. Or direct "Spine Length" from the Customizer window, but animatable?

2. Is it possible to add a buttons for a custom bones? Like SuperBone01... SuperBone03 or something? These doesn't exist in game, but if you make a coat (for example) and add these bones to it with exact names you could animate something with SuperBone01... SuperBone03 and so on.
A proper skirt rigging with these bones and animating it could make clothes look natural.

1) Probably not. There are things we call "Core" issues that can only be changed if we had access to the source code (which we don't.) It's not just a matter of adding buttons to resize the gaps in the spine, but also saving that change in the Pose file AND maintaining compatibility with existing Pose files. Those are all things we can't do simply through scripting.

2) You're describing "morphs". V11 supports morphable clothing. I believe a couple of people have written instructions on how to create "morphable" clothing in Blender.