Step to reproduce issue:
1. Go to composer mode
2. Select any bone to open the bone editor

Actual Result:
The [Insert] button is grayed out.
In other words, template characters cannot add expansion bones between bones.

Expected Result:
In the template character, add an extension bone between bones like free bones.

Additional information:
However, I can't use free bones.
The reason is that there are functions that can only be used with the "template character".

-Motion_Live_Body
https://manual.reallusion.com/Motion_LIVE_2D_Plugin/Resources/CTA_4/Plugins/Motion_Live_2D/Workflow_for_Motion_Live_Body.htm

Due to these functional limitations, it's not a very good idea to create a human character using free bones.

The best fix is ​​to make the above features available for free bones too, but it takes a lot of work.
It will be a long battle to deal with various exception cases.

On the other hand, this proposal only adds a process to ignore the intermediate expansion bone.
As with the end extension bones, you just have to ignore the extra bones.

What you get with this fix:
Improves the expressive power of animations that focus on the upper body.
For example, add extra shoulder bones, like the attached image.