View shed with 3D objects

[JoshWilhelm]

I'm trying to create a viewshed for 3D features that will be constructed on a hillside.  I have created a viewshed from 5 observation points but I need it to take into account the several hundred 3D polygons that I have extruded to a 20' height. I need the viewshed to include these features and not just the DEM that is under the 3D polygons.  Is there a way to add the 3d polygons to the DEM raster, or another tool I can use that will take the 3D features into account with the DEM?  I've also used the 3D viewshed exploratory tool, but need a layer created to show what structures will be visible from the observation point on a map.  

[DanPatterson]

an old school way would be to convert a building footprint to a raster using a height value/field then add that result to the DEM, effectively adding the height to the raster.  (make sure nodata surrounding the buildings is set to 0 )