You cannot receive credit for any military service in your CSRS retirement computation, if you are receiving military retired pay, unless you were awarded the retired pay:
However, you can elect to waive the retired pay and have the military service added to your civilian service in computing your CSRS annuity.
If you want to waive your military retired pay to receive credit for military service in the computation of your FERS or CSRS benefit, you should write the Retired Pay Operations Center at least 60 days before your planned retirement. Send your waiver to:
Defense Finance and Accounting ServiceU.S. Military Retirement PayP.O. Box 7130London, KY 40742-7130
You can "fax" your request to 1 (800) 469-6559.
Suggested wording for your request is as follows:
"I (Full Name and Military Serial Number) hereby waive my military retired pay for Civil Service Retirement or Federal Employees Retirement System purposes effective (The day before your annuity begins). I hereby authorize the U.S. Office of Personnel management to withhold from my (CSRS or FERS) annuity any amount of military retired pay granted beyond the effective date of this waiver due to any delay in receiving or processing this request."