I think the simple answer you are looking for is:
select "File->Shapes->Flowchart->Basic Flowchart Shapes"
when you save this Visio diagram, the stencil will be included when opened
again.
Or if you are asking about stencils included in templates and by "Network
diagram" you mean Basic Network Diagram, then
open the "Basic Network Diagram" template from "File->New->Network->Basic
Network Diagram"
then select "File->Shapes->Flowchart->Basic Flowchart Shapes"
If you want this stencil to be included in the template every time you
create a new one then,
save the blank diagram as a Template (*.vst) and save over "NETW_U.VST" (for
Basic Network Diagram) in the default folder "C:\Program Files\Microsoft
Office\Visio11\1033"
If you are curious about the other templates in this folder, open up the
properties on the .vst file and select the "Summary" tab, you will see the
title Visio uses.
Best Regards,
Joe Mako