We have Belgium Architects and Designers who are using PlusSpec.
With the release of the official version - PlusSpec 2015, you now have full customization, and you can create any type of wall or materials!
I believe that you could indeed import existing walls that will remain unchanged. One of the benefits of PlusSpec is that it utilizes the native SketchUp environment. You have total design freedom - and can model parametrically (with PlusSpec), or 3D sketch using SketchUp. You are not limited by rules - so you can control what you want to show, how you want to show it, and how much time/detail you want to spend on each element.
I have been looking at the Construction types, and none of them really cover this. I either have sarking or I have tophats. The powerpanel system uses two different tophat depths, depending on the R-value you need to achieve.