Hi gunter, ok now I understand better what you're looking for. All my stuff above was talking about making a straight cap...

So yeah putting in a circle and lofting is a good idea. To make it fully smooth, also put a point object in the center of the circle (draw curve > more > point), and then select all 3 of the top curve, circle and ending in the point to do the loft. Then set loft style = "loose" in the loft options. Check the attached 3DM file for an example.

Making a loft that ends in a point object is one of the new features in v3, there are some examples here:
http://moi3d.com/3.0/docs/moi_command_reference7.htm#loft

- Michael