Yes, you would have to create it from another piece.
Or, just an idea: duplicate the Ruled Shell. Define a Contour for both instances. One contour should define the left half, the other should define the right half. The right edge of the left part's contour and the left edge of the right part's contour should coincide.
But if you want to edit that new reference polygon, then you should create it out of 2 ruled shell pieces.
Obviously, the next level for the Shell Tool should be a Shell made up of as many segments as needed with the profile polygon of each editable.
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