I don't know guys,
If the edges all around the end of this shape coplanar, you should just select them and do a regular offset without any further hassle and plugins.
Offset.jpg
Now if they are non-planar, make an intersection with a flat face and do it afterwards.
Another way would be to fill in that face first (just redraw a segment on the perimeter - again, only if the end edges are coplanar) and then do the offset.
Offset2.jpg