There are two types of items displayed in the Widgets section: New Item Forms and New Document Forms. No other Forms or Rules are supported, which means that no other Forms or Rules can be added to the navigation structure within the Design Canvas.
The items displayed in the Widgets section are edited from within the Manage Forms section. Find the Form that matches the name of the Widget and choose to edit. Once the Form is updated, the Widget will also be automatically updated.
Finally, the items displayed within the Widgets section of the Forms & Rules area are those matching the criteria of being New Item or New Document types. Nothing needs to be done to have those Forms displayed in the Widgets; it is automatic.