Cal. Pub. Util. Code § 25305

Nothing in this chapter prevents a district from participating in and making all or part of its employees members of the State Employees’ Retirement System by contract entered into between the district and the board of administration of the system under the State Employees’ Retirement Law or from participating in the Federal Social Security Act, or from participating in the retirement system of any other public agency for which it is eligible, and the district may perform all acts necessary or convenient for such participation. Such participation shall not be considered as establishing a retirement system subject to the provisions of this chapter.