How can rivers associated with state wild and scenic river systems become part of the National Wild & Scenic Rivers System?

For state-designated rivers, a governor may submit an application to the Secretary of the Interior under Section 2(a)(ii) of the Act.  If found eligible, and if sufficient protection is afforded by the state, the Secretary may make the designation.  Rivers designated in this manner continue to be administered by the state (sometimes with assistance from local governments), except for any federal lands along the river.  If there are federal lands located along the river, the state and federal river-administering agencies may enter into an agreement to outline federal/state management roles and responsibilities and/or provide for management and protection of river values.

Source
Interagency Wild & Scenic Rivers Council