Can I retrofit a ground wire so I can install a 4 Prong outlet for my dryer?
Since you have 3 insulated conductors I would add an equipment ground conductor this is legal now. 3 wire systems are still legal and do not require update but it is a good idea to have a 4 wire system this is safer. Make sure the new ground is connected to the ground bus if this is coming from a sub panel and the neutral is on the neutral bus. If in a main panel they may go to the same bus as they are tied together.