The Reagan administration yesterday backdated its tough federal hiring freeze to Nov. 5 and the president announced his choices to head the Internal Revenue Service and to be deputy director of the Central Intelligence Agency.

The decision to backdate the freeze to the day after Reagan's landslide election victory will be a jolt to the bureaucracy and to thousands of people who have been promised federal jobs since then but are not yet on the payroll. The backdated freeze is certain to face legal challenges from disappointed job-seekers.

Except for emergency situations, people offered a job since Nov. 5 cannot now be hired. When no freeze is in effect, the executive branch agencies hire about 1,500 new workers a day.

Reagan chose Roscoe L. Egger Jr., a partner in the giant accounting firm of Price Waterhouse & Co., to head the IRS, and Vice Adm. Bobby R. Inman, an intelligence specialist whom President Carter named to head the National Security Agency, to be deputy to CIA Director William Casey.

Inamn, 49, is a former No. 2 man at the Defense Intelligence Agency and head of the Naval Intelligence Department. As head of the super-secret NSA, Inman established good working relationships with congressional oversight committees. He is regarded within the Pentagon as one of the brightest younger admirals.

Egger, 60, was for 17 years the partner in charge of his firm's tax department before switching in 1973 to head its Office of Government Services.

Reagan promised a hiring freeze during his presidential campaign and issued the order imposing one within hours of being inaugurated. The decision to backdate it is a surprise, however.

The executive branch has been under a partial hiring freeze, allowing only one of every two vacancies to be filled, since last March. Full-time permanent employment dropped by about 20,000 jobs in the first months of this freeze, imposed by President Carter. But recently the agencies -- clearly anticipating a Reagan freeze -- went on what officials have described as a "hiring binge."

Full-time employment in the executive branch has been increasing since September despite Carter's order and now stands at 1.86 million.

Officials at the Office of Management and Budget, which will oversee Reagan's freeze, say that making the freeze retroactive does not mean that people hired in recent months and already on the payroll will be fired. They are probably safe. Those promised jobs but not on the payroll will have to get clarification of their status, OMB officials said.

It is expected that agencies that have issued job promises will have to ask whether they can go through with the hires or not.

The freeze guidelines, signed by acting OMB director Dale R. McOmber (Reagan's choice for director, David A. Stockman, is not expected to get Senate confirmation until Tuesday) say that the freeze applies to "all departments and establishments and to all types of appointments, temporary as well as permanent" except for certain exempted positions.

Exemptions will be for jobs that protect life and property, and for medical, hospital and other health care operations. Agency heads seeking other exemptions must submit written justification to the director of OMB.

The freeze does not apply to the U.S. Postal Service, a quasi-government corporation with more than 650,000 workers, nor to executive-level jobs nor non-career positions in the Senior Executive Service.

Federal officials say a lot of questions remain despite yesterday's new guidelines. They anticipate that some mechanism will be established to handle "hardship cases," such as a person who was promised a job after Nov. 5, quit his old job, sold his home and moved to Washington.

Federal hiring freezes are nothing new. There have been half a dozen ordered by Congress or presidents in the past 20 years, but this is the toughest one in memory and the backdating makes it unique. Critics challenge Reagan's right to backdate a freeze into months when another man was president.

In addition to Egger and Inman, Reagan appointed three other men to top government posts yesterday:

Norman B. Ture, a supply-side economist, to be undersecretary of the Treasury for tax policy.

John M. Fowler, chief financial officer of the Reading Co., to be general counsel to the Department of Transportation.

Robert W. Blanchette, a lawyer with long and varied railroad experience, to be administrator of the Federal Railroad Administration.

In addition, White House chief of staff James A. Baker III announced that John F. W. Rogers has been appointed special assistant for management and acting director of the office of administration in the White House. Rogers will be responsible for managing personnel, finance and correspondence units in the White House as well as for the White House Visitors Office.

Reagan held a Cabinet meeting yesterday, but promised that Saturday Cabinet meetings would not be frequent in his administration. The Cabinet was briefed by Secretary of State Alexander M. Haig Jr. on the emotional health of the returned Iran hostages and the plans for welcoming them to the United States, and then the bulk of the meeting was devoted to the economy, according to White House Press Secretary James Brady.

Although Reagan held a Cabinet meeting every day during his first week, only one is planned this week and the work on Reagan's economic package will be carried on in smaller "budget working groups," Brady said.

Reagan left the White House yesterday for lunch and dinner to visit two of Washington's old establishment clubs. He had lunch at the Alibi Club and dinner at the Alfalfa Club.