Mo. Rev. Stat. § 476.684
Any person having at least four years of service as a judge, as defined in section 476.515, and who had at least ten years of prior service as a state employee prior to September 1, 1959, may reinstate such service to receive prior service credit therefor if the person shall pay into the fund an amount equal to any refund of contributions which the person may have received, with simple interest thereon, to the date application is made for such prior service credit. The person shall then become a member of the Missouri state employees' retirement system. Thereafter not more than fifty percent of such credit may be transferred to and be counted as service as a judge for the purpose of calculating retirement benefits for judicial service if retirement benefits as a judge have not yet begun.
§ 476.684, RSMo