Notwithstanding any other provisions of this part, the board shall execute a modification of the agreement in conformity with the provisions of Section 218 of the Social Security Act and applicable federal regulations adopted pursuant thereto, to include the services of employees in positions covered by the Judges’ Retirement System, when a majority of the eligible retirement system employees in positions covered by such system at a referendum conducted by the board pursuant to Article 2 of this chapter and Section 218(d) of the Social Security Act have voted in favor of inclusion in the federal system.