S.C. Code § 38-77-180
If the owner or operator of any vehicle causing injury or damages by physical contact is unknown, an action may be instituted against the unknown defendant as "John Doe" and service of process may be made by delivery of a copy of the summons and complaint or other pleadings to the clerk of the court in which the action is brought. The insurer has the right to defend in the name of John Doe. However, the bringing of an action against the unknown owner or operator as John Doe or the conclusion of that action does not constitute a bar to the insured, if the identity of the owner or operator who caused the injury or damages complained of becomes known, from bringing an action against the owner or operator previously proceeded against as John Doe. Notwithstanding the uninsured motorist provision nor any other provision of law, the joinder of any other person causing the injury as a party defendant, in an action against John Doe, is allowed.
S.C. Code § 38-77-180