Cal. Gov. Code § 21032
A member may elect at any time prior to retirement, in accordance with regulations of the board, to receive credit for public service, under any of the definitions in this article, in addition to his or her current and prior service credit. An election shall be effective only if accompanied by a lump-sum payment of the contributions and interest required for the credit or by authorization for immediate institution of payroll deduction of installment payment of the contributions and interest. The right of election is subject to Sections 20894 and 20961, and shall be ineffective with respect to any time or employment for which the member subsequently becomes entitled to or eligible to elect to receive service credit in another system supported in whole or in part from public funds, in which case accumulated contributions on deposit for the period of service credit shall be paid to the member.
Ca. Gov. Code § 21032