Section 40 ILCS 5/18-112.5 - Payments and Rollovers(a) The Board may adopt rules prescribing the manner of repaying refunds and purchasing any optional credits permitted under this Article. The rules may prescribe the manner of calculating interest when such payments or repayments are made in installments.(b) Rollover contributions from other retirement plans qualified under the U.S. Internal Revenue Code may be used to purchase any optional credit or repay any refund permitted under this Article.