Revit creates shadows on the fly using the graphics card, so shadows represent what you can see on the screen.
So it is not possible to separate out shadows from existing buildings and new buildings in the same view.
The only way is to trace over existing shadows with lines or filled regions (existing shadows won't change as the design changes).
Create a view that only shows existing buildings and objects.
Turn on shadows.
If it doesn't exist create a filled region called .Shadows-Existing, give it the properties you want.
If it doesn't exist create a Linetype called .Shadows-Existing, give it the properties you want.
Trace shadows with using the .Shadows-Existing filled regions and .Shadows-Existing linetype.
Zoom in close because you will be doing by eye - you can't use snaps.
When finished Copyclip all the shadow filled regions.
Paste aligned into each shadow diagram view.
If you want to change the appearance of existing shadows change the properties of the .Shadows-Existing filled region type and/or the .Shadows-Existing linetype.