If I create my own tool and add it to the modify features pane using the L_group attribute, I then see my tool all along in there. However some tools seem to have companions. For example, when I employ the Move tool, After selecting a feature to move, the dialog shows three tools at the bottom: Move, Rotate, and Scale. How are Rotate and Scale getting added in there with Move?
The three tools - Move, Rotate and Scale have a custom UI that links them together that is part of the internal Editing code. Unfortunately it is not something that is customizable via the daml of an add-in.