You typically do not need to add soil to install sod unless it will help grade the area for drainage purposes. If you want to amend the soil where you are installing the grass then we would recommend using compost and mixing it into the existing soil. The other option is to use a good garden soil. Some garden soils have wood waste in them and can lead to other issues later down the line. Topsoil is always an option but there is no guarantee that there will not be weed seeds within.