To combine two or more diagrams (process levels) into a new single one, do the following:
Select diagrams in the diagram drawing area (hold the Shift key down while clicking the desired diagrams)
Right-click any of them to open the pop-up menu..
Select Combine diagrams > horizontally/vertically according to way you want to combine them. A new diagram is created containing the contents of all selected diagrams and they are aligned according to the way you selected.