Theoretically, Toolbox is a container (or a container for the container). And you can add a child Widget to it. So you will have to make a button dynamically and append it to toolbox. However, I am not sure how it will work.
You need to figure out what Kind of objects Krita uses for Tool Buttons. It might not be exposed to Python API.
If you considering paid options the Actuator plugin is something you can use to make your own tool panel.