I don't thin you can do that. Either use sketch entities (lines, arc, etc.) in drawing to generate data for block or save your drawing as dwg and import back to SW drawing and make the block.
I have the same question.
I just export the drawing view to DWG or DXF and import it back to a sketch and then make the block.
It costs lots of time because of the huge amount of elements. With conversion sometimes arcs, splines etc. are not completely converted.
Do you have any other info on this in the meantime?
with all kind regards.