Mo. Rev. Stat. § 70.638
Any employee of a county, including any elected county official, who became a member of the system due to the amendment to the definition of "employee" in section 70.600, and who has been previously employed by a county in a position which is now considered covered employment, may elect to purchase all of his membership service or prior service for which he would have been required to make member contributions provided for in section 70.705 if the member provides an affidavit stating that he is not receiving and is not eligible to receive retirement credits or benefits from any other public or private retirement plan for the period of service to be purchased. The purchase shall be effected by the member's paying to the members deposit fund the amount required in section 70.705 together with regular interest from the date of employment to the date of repayment. Each political subdivision shall also be responsible for making employer contributions in accordance with section 70.730.
§ 70.638, RSMo