5 CFR § 1620.13 - Retroactive contributions.

(a) An employing authority can make retroactive employer contributions on behalf of FERS employees described in this subpart, but cannot duplicate employer contributions already made to the TSP.

(b) An employing authority making retroactive employing agency contributions on behalf of a FERS employee described in § 1620.12 must continue those contributions (but only to the extent they relate to service with the employing authority) if the employee returns to his or her agency of record or is transferred to another Federal agency without a break in service.

(c) CSRS and FERS employees covered by this subpart can make retroactive employee contributions relating to periods of service described in § 1620.12, unless they already have been given the opportunity to make contributions for these periods of service.