5 CFR § 831.403 - Eligibility to make voluntary contributions.
(a) Voluntary contributions may be made only by—
(1) Employees (including phased retirees) or Members currently subject to CSRS, and
(2) Applicants for retirement, including phased retirees who apply for full retirement status under subpart Q of this part.
(b) Voluntary contributions may not be accepted from an employee, Member, or applicant for retirement who—
(1) Has not deposited amounts covering all creditable civilian service performed by him or her; or
(2) Has previously received a refund of voluntary contributions and who has not been reemployed subject to CSRS after a separation of more than 3 calendar days.
(c) An employee or Member covered by the Federal Employees Retirement System (FERS), including an employee or Member who elected to transfer or was automatically placed in FERS, may not open a voluntary contributions account or make additional contributions to an existing voluntary contribution account.