Federal workers who want to increase their civil service pensions by making interest-free contributions covering their years of military service are getting a year's extension.

The government allows employes to credit military service toward their civil service retirement, and hundreds of thousands have elected to do so.

Interest was supposed to be added starting Sept. 30 to any payments made for post-1956 military service.

But because of a change in federal procedures, that deadline will be extended, and no interest will be charged on retirement fund deposits for military service if they are made by Sept. 30, 1986.

At age 62, those who are eligible to receive Social Security benefits automatically lose civil service pension credit for any time after 1956 spent in the military. It's known in the federal service as "Catch-62," for the age at which benefits are recomputed.

The law was enacted to cut out double-dipping by persons who would be eligible for Social Security as well as civil service benefits increased by the addition of military service.

Several years ago Congress changed the law to make it possible for veterans to avoid the Catch-62 recomputation.

Those who chose to do so may "buy" into the civil service retirement fund by paying 7 percent of their post-1956 military income into the civil service retirement fund before they retire.

The law said that interest (currently set at about 13 percent) would not be charged if those payments were made by Sept. 30 of this year.

But because of a little-noticed change in the way government computes interest payments, the interest-free period for making payments will run until the end of the next fiscal year, Sept. 30, 1986.

Contributions still may be made after then, but interest will be charged.

Office of Personnel Management officials say that federal personnel offices and payroll offices were notified of the change, but because of the complexity of the issue, many didn't advise workers.

The information was published last week in the Federal Employees News Digest, prompting a rash of calls from veterans to their personnel offices.