NURBS Tubes: How to close the ends before converting to Mesh?

I created a “tube” of varying sizes and shapes throughout its “extrusion”.

(I have deformed NURBS circles along the z-axis, one on top of another, for example).

I want to close the top and bottom ends to have nice and smooth “caps”.

How do we do that?

ALSO, if ever we convert to Mesh, the top and bottom end holes are concave, so the Extrude-0 Scale-0 trick won’t work, and there are way too many vertices on the edge to create faces, one polygon at a time.