A member may purchase service credit as provided in subsection (b) of this section for periods of employment for which the member: would have been entitled to purchase service credit under the Employees’ Retirement System; and is not otherwise entitled to service credit under the Correctional Officers’ Retirement System; or was not vested in the Employees’ Pension System. A member who purchases service credit under this section shall: complete a claim for the service credit and file it with the Board of Trustees on a form that the Board of Trustees provides; and pay to the Board of Trustees in a single payment the member contributions the member would have made for the period of employment for which service credit is being purchased plus regular interest to the date of payment. A member may pay for service credit purchased under this section at any time before retirement.