A morph can only move the existing vertices around, it can’t add or remove points nor can it rearrange them (by cutting a hole, say). If you want a higher detail modification you need to use displacement - the usual technique would be to add your extra sub divisions (with smoothing off, otherwise some areas - such as the teeth - will get unwanted modifications), create your shape, then dial the smoothing right down and send the morph back to DS, then use the grouping tools in ZBrush to split the mesh into subtools by material groups (this is easiest using an OBJ exported from DS and modified in a modeller to group the material zones by uv map, rather than using the Bridge) and export displacement maps to give the finer detailing on top of the morph.