There are many reasons to use an under-sculpt; to hold the clay together while it is baking, keep delicate parts from breaking, control the thickness of clay on the figure, and to help you control the difficult to work with polymer clay. An under-sculpt has the added advantages of reducing the final baking time and making the figure stronger.
Think of your wire armatures as the doll’s skeleton. The under-sculpt is applied over the wire armature. The under-sculpt can be anything to bulk out the thick parts of the figure and to firm up the thin wire armature and will be compatible with polymer clay and baking temperature heat. Materials to use to fashion an under-sculpt are polymer clay (bake hard) and modeling epoxy (air dry). Some doll makers use hard packed aluminum foil.
This link will take you to a 3 page photo tutorial demonstrating an under-sculpt: