Cal. Fam. Code § 17428
In any action or judgment brought or obtained pursuant to Section 17400, 17402, 17404, or 17416, a supplemental complaint may be filed, pursuant to Section 464 of the Code of Civil Procedure and Section 2330.1, either before or after a final judgment, seeking a judgment or order of paternity or support for a child of the mother and father of the child whose paternity and support are already in issue before the court. A supplemental judgment entered in the proceedings shall include, when appropriate and requested in the supplemental complaint, an order establishing or modifying support for all children named in the original or supplemental actions in conformity with the statewide uniform guideline for child support. A supplemental complaint for paternity or support of children may be filed without leave of court either before or after final judgment in the underlying action. Service of the supplemental summons and complaint shall be made in the manner provided for the initial service of a summons by the Code of Civil Procedure.
Ca. Fam. Code § 17428