What is the ruling on using hair fillers made of nylon or sponge to give hair more volume? Does the ruling differ for someone suffering from baldness or thinning hair? Is it permissible for a woman suffering from partial baldness to wear a wig in front of her husband and female friends?

There is no objection to placing hair extensions within the hair, especially if there is a need to remove the disfigurement of baldness. And if it is assumed that it is connected to the hair, that is not forbidden, because what is forbidden is connecting hair with hair for deception and using impure hair, which does not apply to connecting non-hair to it.