Mo. Rev. Stat. § 86.143
In addition to the contributions deducted from compensation as herein provided, any member may redeposit in the annuity savings fund by a single payment or by an increased rate of contribution an amount equal to the total amount he previously withdrew therefrom as provided in sections 86.010 to 86.193 or any part thereof or any member may deposit therein by a single payment or by an increased rate of contribution an amount computed to be sufficient to purchase an additional annuity which together with his prospective retirement allowance shall provide for him a total retirement allowance of not to exceed one-half of his average final compensation at age sixty. Such additional amounts so contributed shall become a part of his accumulated contributions except in the case of ordinary disability retirement when they shall be treated as excess contributions returnable to the member in cash or as an annuity of equivalent actuarial value. The accumulated contributions of a member withdrawn by him or paid to his estate or designated beneficiary in event of his death shall be paid from the annuity savings fund. Upon the retirement of a member his accumulated contributions shall be transferred from the annuity savings fund to the annuity reserve fund.
§ 86.143, RSMo