Civil servants from postal clerks to civilian astronauts could, collectively, become a source of billions of dollars a year in new stock market and securities investments under a federal pension program Congress is expected to approve this week.

President Reagan has agreed to sign legislation authorizing the new pension system as soon as it passes the House and Senate.

Benefits would be based on Social Security, a modified civil service pension and tax-deferred deductions invested while working. The investment plan is similar to the so-called 401(k) savings options available to many private- industry workers.

The civil servant investment provision would be subject to any limits Congress may put on tax-deferred investments as part of tax reform.

If the pension legislation passes, a large chunk of the government's $80 billion annual payroll could be diverted to tax-deferred investments within a few years.

Workers hired since January 1984 would be automatically covered by the new pension plan. The 2.4 million employes hired before that time could stay under current civil service coverage, or decide between July 1 and Dec. 31 of next year whether to join the new plan.

Under the proposed investment program, up to 15 percent of pay could go into one of three investment options. Two are government securities and programs with fixed rates.

The third, a higher-risk option, would allow investments in the stock market. The legislation would set up a board to manage the program.

Employes could defer taxes on up to 10 percent of their salaries each year by investing in one of the options. The government would match up to 5 percent.

Even workers who put none of their own money into the investment plan would have the equivalent of 1 percent of their salaries put in for them by the government. All contributions and earnings would be tax-deferred until employes retired or left government and withdrew the money.

Federal workers who chose to stay in the current civil service pension plan would be allowed to participate in the tax-deferred investment program on a limited basis. They could put up to 5 percent of their salaries into government securities. There would be no matching contributions from government.

The retirement bill would end the extra 5.7 percent retirement deductions the government started taking out of paychecks May 1 for employes hired since January 1984. Those employes would be reimbursed. Meetings

Former labor secretary William J. Usery Jr. will speak at the May 28 dinner at the Touchdown Club of the Industrial Relations Research Association. Usery will talk about labor-management relations. For details call Mary Green Miner at 452-4130.

National Association of Retired Federal Employees Annandale chapter will have a luncheon meeting June 6 at the Country Club of Fairfax. For information call William White at 978-8081.

National Association of Government Communicators will hold an award banquet June 3 at the National Press Club. ABC White House correspondent Sam Donaldson is scheduled to speak. For details call 823-4821. Overseas Jobs

An Army civilian recruiting team will be at Fort Belvoir on June 12 looking for people to work in Heidelberg, West Germany. Army needs social workers, recreation specialists, occupational and physical therapists and other health professionals. Most jobs require civil service status. For details call Shelbie Casey, 664-2546.