It would be nice to be able to add an extra layer of material to a wall type already defined, so you don't have to create a whole new wall type if you just need it in one particular area. For example, you might add a layer of gyp board to wall that's already in the model w/o having to create a new wall type.
This would be useful on some of my projects where I might have a 2-hour wall around a stairwell, w/ 2 layers of gyp board on both sides, and then I have another non-rated wall butting up to the ends of one of the 2-hour walls. To simplify construction the stud layers need to align, and then I could just add an extra layer of gyp board to that non-rated wall to make the finish faces align, without having to make a whole new wall type just for that one instance.