Hi ed - one general way to put a roundy end on something like that is to split the end into 2 edges and then use Construct > Blend to put a blend surface between those edges. It will not be an exact sphere but you can adjust it using the Bulge slider option in Blend.

If you want an exact sphere you can draw in a sphere and trim it, or possibly it might be quicker to select the end circle edge, run Construct > Revolve, put in an angle of 180 and uncheck "Cap ends", then draw the revolve axis across the circle, that should produce a half sphere directly out of the revolve command.

- Michael