NEW: Special counsel Jack Smith charges former Pres. Trump with additional crimes of obstruction of Justice and willful retention of classified documents in the Mar-a-Lago case, in a superseding indictment.
NEW: Hunter Biden tells the court in his plea hearing that he will plead guilty to all charges.
DEVELOPING: The hearing begins without the judge addressing claims a staffer at the firm representing Hunter Biden lied about being a lawyer for a GOP lawmaker in an effort to get a court filing removed from the docket.
It's possible the matter can come up later in the hearing.