Any county officer elected by vote of the people who has elected to participate in the Fund may transfer to this Fund credits and creditable service accumulated under any other pension fund or retirement system established under Articles 2 through 18 of this Code, upon payment to the Fund of (1) the amount by which the employer and employee contributions that would have been required if he had participated in this Fund during the period for which credit is being transferred, plus interest, exceeds the amounts actually transferred from such other fund or system to this Fund, plus (2) interest thereon at 6% per year compounded annually from the date of transfer to the date of payment.