California Code, Penal Code - PEN § 908.1

When, after the grand jury consisting of the required number of persons has been impaneled pursuant to law, the membership is reduced for any reason, vacancies within an existing grand jury may be filled, so as to maintain the full membership at the required number of persons, by the jury commissioner, in the presence of the court, drawing out sufficient names to fill the vacancies from the grand jury box, pursuant to law, or from a special venire as provided in Section 211 of the Code of Civil Procedure.  A person selected as a grand juror to fill a vacancy pursuant to this section may not vote as a grand juror on any matter upon which evidence has been taken by the grand jury prior to the time of the person's selection.