The 400,000 federal workers who took a 5.7 percent pay cut May 1 -- when an added pension deduction went into effect -- could get some good news today if the House approves a new retirement system for them.

Following approval, one of the first orders of business will be to instruct federal agencies to stop the extra withholding. Those deductions have just begun to show up in the checks of nearly 60,000 workers here.

The Senate approved the pension program Monday evening, and President Reagan has promised to sign the final version.

The new plan, modeled after private sector pension programs, bases retirement benefits on Social Security, a modified civil service pension and earnings from an optional tax-deferred investment program.

Starting in 1987, all employes hired since January 1984 will be automatically covered by the plan. Workers hired before 1984 can stay in the current civil service retirement program or elect to join the new system between July and December of next year. Potomac Forum Seminars

The Potomac Forum is running two separate workshops Wednesday through May 30 on automatic data processing procurement policies, and with the government's contracting-out (A-76) program. The sessions will be held at the Hyatt Crystal City Hotel. For details call 883-2200. Meetings and Dinners

The Classification and Compensation Society and American Association for Budget and Program Analysis will hold a joint luncheon Wednesday at Blackie's House of Beef. Call 783-4847.

The Presidential Management Interns Alumni Association will hold its public service awards banquet Wednesday evening at the National Press Club. Awards will go to Deborah Chassman of the Department of Health and Human Services and Charles Adolph of the Air Force.

The Maryland chapter of the Senior Executive Associaton has scheduled a retirement planning seminar dinner for June 19 at the Holiday Inn in Laurel. For information call Marsha Kemp (301) 594-1411.

The Interagency Advisory Group's drug and alcohol abuse committee meets at 10 a.m. today, at the Office of Personnel Management. The topic is cocaine. Awards

General Services Administration's Ron Gabriel has been given the writers award by the International Personnel Management Association's Eastern Region.

Government Employees Insurance Company has announced that it will once again sponsor its public service awards program. It recognizes federal employes and retirees for their work or community efforts in fire and traffic accident prevention, physical rehabilitation and alcoholism. Winners are honored at a banquet and each gets a $2,500 check.

Nominations are due by Sept. 1. To nominate federal employes, write to Geico Public Service Awards, Nomination Committee, Geico Plaza, Washington, D.C. 20076. Retiree nominations should go to the National Association of Retired Federal Employees (Geico Awards Committee), 1533 New Hampshire Ave. NW, Washington 20036.