Polyline manipulation

[447Debbie]

After some trial and error, I've finally succeeded in getting my topographic elevation lines imported into Home Designer Pro.  The terrain perimeter is the big rectangle, which actually covers the area outside of the actual lot.  That's OK, and probably even a good thing, BUT, I'd like to somehow define the lot polygon so that I can see it in 3D perspective view as I'd like to look at this in 3D but still be able to tell where the actual lot line is.  The lot is the gray highlighted area in the attached picture.  It is defined by a polyline.  Is there a way to make that polyline a fence or something in order to be able to see this in the 3D perspective view?