I'm kind of surprised that this query didn't get answered. But after trawling through lots of posts it looks like it simply isn't possible in 3DGS. For others who may find the same query solution useful I will try to explain here:

Basically, use a PH_HINGE between the walls/panels that you want to use to create the concave shape (see figure 1). Set the swing angle for the hinge to -1/1 - so it can't move much. But you need to avoid internal collisions with other parts of the car body - so leave a gap between each entity. Then make the walls or panels INVISIBLE and use the addshape for the actual car body (see figure 2).