Let's follow the steps for this.
1. There are a shortcut to be added into additional styles. 'Gallery' is the shortcut in the following screenshot.
2. Long-clicking the shortcut will show up the popup menu.
3. Press 'save' button. Then the shortcut will be added as an additional shortcut style.
You can find it in the 'ADDITIONAL' tab of the style selection screen.
4. Let's create a new shortcut and apply the additional style to it. Of course, the target of the new shortcut will not changed because it applies only 'style'.