Yup, this inability to add skins was a much-blasted non-feature when renovation was introduced.
The Graphisoft workaround shows that their design team is just not thinking as one because not only is it not a reasonable workaround - it also directly conflicts with the new energy modeler in 16:
Adding an entirely new wall for the added skin (e.g., insulation) means putting empty openings in it all over the place to match existing windows/doors ... and of course, the jambs and casing will not work at all.
Further, adding a new wall for the added skins prevents the new energy modeler from working properly! The new modeler (perviously EcoDesigner) only works with a single solid or composite wall. Being zone-based now, the wall is only recognized if a zone surface touches the wall. It is thus impossible to get a proper energy solution with two (or more) parallel composite walls ... which is the Graphisoft solution to renovations involving skin changes.
Very disappointing on both fronts.
PS On the jamb/casing business: that's sort of the same type of subatomic control we need for proper renovation scheduling, too - just like your skin example. Depending on which side of the wall you're extending via skin changes, you might remove existing trim and add jamb extensions .. and then either replace existing trim, or install new trim. But, library parts are indivisible things with no concept of having only part of them tagged as demo/new... So, I'm not really sure how one is supposed to schedule such things in a true, BIM way.
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