The interest paid in the voluntary contribution retirement savings program available to CSRS (but not FERS) employees will rise from 2.125 to 2.75 percent. That rate also applies as the interest rate for required deposits and redeposits into the retirement fund to get credit for service for which no contributions were taken or for which refunds were received at a break in service, as well payments as to capture credit toward federal retirement for military service time, for those eligible.

CSRS and CSRS Offset employees may make voluntary contributions to the retirement fund and earn market interest rates (of around 2 percent in recent years) tax-deferred. However, voluntary contributions may only be made if you do not owe a deposit or redeposit to the retirement fund.
Read more on Voluntary Contributions Under CSRS/CSRS Offset at ask.FEDweek.com