One way...
Convert that building to a raster using the height field to assign a value to the building (NOTE set the surrounding area as zero)?
You can then add the building raster with the dem and your building will appear as an "extruded" feature in the terrain
You can visualize your building polgyons as 3D extruded features using the extrusion control on the appearance tab of the ribbon: https://pro.arcgis.com/en/pro-app/latest/help/mapping/layer-properties/extrude-features-to-3d-symbol...
I'm not sure if you can use extruded polygons for viewshed analysis, but if not, you can convert the extruded polygons into multipatch features using this tool: https://pro.arcgis.com/en/pro-app/latest/tool-reference/3d-analyst/layer-3d-to-feature-class.htm