Present federal workers would be allowed to keep their retirement system, but all new government hires would be put under social security if Congress accepts recommendations of a blue ribbon retirement study group.

Proposals of the Social Security Advisory Committee, if they become law, would allow incumbent federal and postal workers -- including more than 350,000 here -- to stay under the civil service retirement plan. Or they could elect to switch to social security with benefits supplemented by their "staff" program and regular social security benefits.

The changes are part of a far-reaching blueprint for change in the nation's biggest pension-welfare system. All must be approved by Congress.

Experts anticipated the proposal to "grandfather" civil servants into their retirement program. To force them into merger, they felt, would have been a fiscal and political nightmare. Groups respresenting federal and postal workers and retirees have already raised $3 million to fight mandatory coverage in the next Congress.

But committee members believe it would be practical, and desirable, to require new federal workers to come under social security, and to work out credit transfers for present employes who want to come in voluntarily. Mandatory social security coverage would affect only workers hired after the date of enactment of legislation authorizing it.

The committee said there were two other options -- which it did not recommend -- for extending social security coverage to federal, state and local government workers immediately. They are:

Mandatory coverage of all future and most present employes with "some advance notice and some exclusions for those near retirement age."

An exchange of credit plan "patterned after the current social security-railroad retirement interchange. Under this approach a beneficiary would receive benefits based on his combined earnings under social security and the staff retirement system. The benefits would be paid under one system only, and credits would be transferred from the system not paying benefits to the one that is."

Most federal, postal and retiree groups oppose any type of mandatory coverage. Although they doubt Congress will act during the 1980 election year, they do not like even the mention of mandatory coverage.

Most of the organizations see the committee proposal for future mandatory coverage as a foot-in-the-door that must be stomped with vigor. They worry that mandatory coverage could eventually undermine the civil service retirement system.

With no new people coming into it, they fear the civil service program could wither away. Congress, they fear, could lose interest in funding a pension system with no new people entering it, and in time could begin to cut back promised benefits to the shrinking group.

Backers of universal coverage argue it is fair to the taxpayers and could result in better benefits for federal retirees. They say benefits for short-term workers killed or disabled would be better under social security than under the federal staff retirement program.