40 ILCS 5/14-104.11
An employee may establish creditable service for periods prior to the date upon which the Illinois Finance Authority first becomes a department (as defined in Section 14-103.04) during which he or she was employed by the Illinois Finance Authority or the Illinois Industrial Development Authority, by applying in writing and paying to the System an amount equal to (i) employee contributions for the period for which credit is being established, based upon the employee's compensation and the applicable contribution rate in effect on the date he or she last became a member of the System, plus (ii) the employer's normal cost of the credit established, plus (iii) interest on the amounts in items (i) and (ii) at the rate of 2.5% per year, compounded annually, from the date the applicant last became a member of the System to the date of payment. This payment must be paid in full before retirement, either in a lump sum or in installment payments in accordance with the rules of the Board.
40 ILCS 5/14-104.11