Federal workers, who enjoy one of the world's best staff retirement programs, would have to set aside much more of their paycheck - or take smaller pensions - if forced to convert to Social Security.

That is the tentative conclusion of experts in and out of government who are studying options Uncle Sam has if it is decided to merge the Civil Service Retirement System with Social Security. Currently the typical federal retiree gets a taxable annuity of $634 a month. That is more than double the average Social Security benefit which is, however, tax-free.

Legislation before Congress would bring postal, federal and uncovered state and local government employes into the Social Security system. It covers most other American workers and not pays benefits to one of every six persons in the United States.

Several blue-ribbon study groups are considering mandatory social security coverage for civil servants. The primary one, the Universal Social Security Coverage Study, is due to make its final report to Congress in December. President Carter says he has no position for, or against, mandatory coverage, and will not make up his mind until he reads the USSC report. Although legislation is pending in Congress to bring federal workers under Social Security, it has little chance of passage this year. It does almost nothing to explain how the massive changeover would be accomplished, who would be exempt from it and how benefits would be paid.

But hints are coming from the various study groups as to what their recommendations will be.

Based on a look at a mid-term report, talks with Administration officials, retiree group leaders and legislators briefed on the options, the proposal may be something like this:

- Mandatory social security coverage for federal and postal workers will be recommended.

- Advisers guess that President Carter will go along with it.

- The proposal would not, as many hope, "grandfather-in" all present federal and postal workers. Rather, it would allow only workers with vested rights (of 5 or more years in the civil service system) to remain under the federal retirement program.

- Short service employees without vesting, and all new hires in government would automatically go under social security.

Supplementary staff retirement programs would be developed in government. But federal and postal people would have to pay into social security, and extra for their own staff retirement program. (More on how that would work in tomorrow's column.)

- The Civil Service retirement fund assets would NOT be merged with Social Security. Many employes believe their fund, which is actuarially sounder than social security, is coveted to help pump up social security and pay out benefits. However, under proposals which seem to have the most support, the two funds would not be comingled.

- Government workers (who generally can retire earlier on larger annuities than under social security) would still be allowed early-outs. But their immediate monthly retirement incomes would be reduced to the amount of their supplemental civil service benefits. They would not qualify for social security until they reached regular ages for it.

- President Carter still has an open mind on the issue of mandatory or universal coverage.

- No present retiree would lose benefits under merger.

Many federal and postal unions have charged that the social security-civil service merger, which they oppose, is being master-minded by Jimmy Carter. Nothing could be further from the truth, according to aides who have worked with the president on the issue.

"Believe me, his mind is open on this subject," one senior civil servant said. "He (the president) didn't even know about the proposed merger until a few weeks ago when we briefed him on the progress of the study groups. He's interested, and seems to have grasped the concepts. But he hasn't said yes, no, or even maybe."

Several weeks back President Carter met with 13 handpicked civil servants to talk about government and their jobs. Aides briefed him on questions that might come up. One of them dealt with social security coverage.

"We were very careful to get him to use the key phrase that nobody in the system would lose any benefits" an aide said.