There are two file formats that you can download from Vans, DXF or IGES. In either case you end up with the outline shape only, not a parametric model. If you have created a .SLDPRT Solidworks Part, then all you have to do is create an Extruded Boss/Base to make the extension you want. I would create it across the bottom straight edge so that you can make the 90 degree bottom flange in one operation.