Creating Projects Using Structure Planning 
To create a project using structure planning, proceed as follows:
The Create project dialog box appears.
You can use an existing project or a standard project as a template.
The screen Change: Structure Planning appears.
The WBS elements, activities, and activity elements are created.

You cannot create subnetworks in the WBS, though you can have the system indicate whether an activity has subnetworks.
See also:
Work Breakdown Structure (WBS) Network