It is possible to add content to parent document while editing child document in place by drag and dropping an element from the Library pane to the Hierarchy pane.

Steps:
1- Create document in the builder and add some content to it
2- Save it
3- Create another document
4- Add an instance of step's 1 document to it
5- Right-click on the instance and edit it in-place
6- From the Library pane, drag and drop a button onto the Hierarchy view

Actual outcome:
The button is added to the parent document

Expected outcome:
The button can only be added to the document being edited