40 ILCS 5/9-121.16
A person who has rendered continuous contractual services (other than legal or actuarial services) to the Retirement Board for a period of at least 5 years may establish creditable service in the Fund for up to 10 years of those services by making written application to the Board before July 1, 2003 and paying to the Fund an amount to be determined by the Board, equal to the employee contributions that would have been required if those services had been performed as an employee.
For the purposes of calculating the required payment, the Board may determine the applicable salary equivalent based on the compensation received by the person for performing those contractual services. The salary equivalent calculated under this Section shall not be used for determining final average salary under Section 9-134 or any other provisions of this Code.
A person may not make optional contributions under Section 9-121.6 or 9-179.3 for periods of credit established under this Section.
40 ILCS 5/9-121.16