It's easy to get to the "notebook view" and once there you can "right click" aka control-click to open a notebook inside a notebook
You can just do step one below then drag an existing notebook to the stack you want, or do this:
tap Command-option-2 to go to notebook view
select the notebook where you want to create a stack, or the stack where you want to locate the new notebook
control click, on that stack or notebook
choose the first option "Create a new notebook in...."
to exit notebook view just click anywhere in the sidebar
I agree a better user experience would allow one to move a notebook to a stack in the sidebar; that SEEMS like how it should work.