In the compositing tool i need the mask as a single object.
If i export the alpha channel i.e. from an png-export out of AS, then the "alpha-mask" has an depth of 8 bit.
You could put all your layers inside a group layer, and then apply a shadow to that. Or play around with the layer blending mode. Must be a number of ways to do it.
You can't have everything. Where would you put it?