Two key unions at the Kennedy Center have accepted a wage freeze, enabling the Washington Opera to present a full seven-production, 63-performance season in 1983-84, general director Martin Feinstein announced yesterday at a press conference in his office.

The orchestral musicians and stage hands who work for the company have agreed to sacrifice salary increases of approximately 9 percent, which already had been negotiated and agreed upon, Feinstein said. The savings in wages, estimated at between $65,000 and $70,000, will allow the company to present seven opera productions rather than six, with performances running from Nov. 5 through Jan. 22, 1984.

New productions will include Verdi's "Rigoletto," Offenbach's "La Belle He'le ne," a double bill of Menotti's "The Medium" and "The Telephone" and Mozart's "Cosi fan tutte," the last a joint production with the Orchestre de Paris. Returning productions will be Handel's "Semele," Rossini's "La Cenerentola" and Donizetti's "L'Elisir d'Amore."

Sam Jack Kaufman, president of the musicians union, Local 161-710, said the wage freeze had been accepted because "our members appreciate and trust Martin Feinstein." Turning to Feinstein, he added: "We've never found you lying to us; you laid it out on the table the way it is, and a decision had to be made."

A press release quoted Jack Ryan, president of Local 22 of the stagehands union, as saying, "We hope this will provide more employment for everyone concerned."

Feinstein announced that the company sold 92 percent of its tickets last season and predicted the Washington Opera will break even next season.

The production made possible by the wage freeze will be Offenbach's "La Belle He'le ne," opening Nov. 21 for five performances, Feinstein announced. The remainder of the 1983-84 season is carefully balanced between three new productions and three returning from earlier seasons. Between the lines can be detected an artfully cultivated and concealed austerity--in the use of shared productions, the recycling of productions for which the sets and costumes already are available, and the choice of several productions that will require a relatively small number of singers.

The season will open Nov. 5 in the Opera House with "Cosi fan tutte" conducted by Daniel Barenboim. On Nov. 7, "Rigoletto" will have its opening night with the noted Swedish baritone Ingvar Wixell in the title role and Elizabeth Knighton in the role of Gilda. Wixell and Hungarian tenor Denes Gulyas will be making their Washington Opera debuts in this production; Knighton appeared here last season.

The Menotti double feature is scheduled to open Dec. 19 in the Terrace Theater. It will be directed by the composer, who also will repeat his direction of "La Cenerentola," opening Dec. 31 in the Terrace. "Semele" will open Nov. 28 and "L'Elisir d'Amore" the next night, both in the Terrace.

Besides Knighton, singers returning from earlier seasons will include soprano Martha Senn in "Semele," basso Franc,ois Loup in "L'Elisir d'Amore" and "Cenerentola," tenor Michael Ballam in "La Belle He'le ne" and mezzos Zehava Gal and Joanna Levy in "Cenerentola." John Reed, formerly of the D'Oyly Carte Company, who sang in "Trial by Jury" last season, will return in "La Belle He'le ne," which will be sung in English. Newcomers to the Washington Opera will include Richard Bauer of Arena Stage in a special guest appearance in "La Belle He'le ne."

Besides Barenboim, conductors will include Cal Stewart Kellogg and John Mauceri, familiar from earlier seasons, and Nicholas McGegan and Lorenzo Muti, who will be making their debuts with the company.

The press conference began without chairman of the board David Lloyd Kreeger, who had phoned in to say he would be delayed because he had been stopped for speeding.

"Impossible," commented G. William Miller, former secretary of the treasury, who now is a member of the Washington Opera's board in charge of corporate fund-raising. "Nobody coming to this press conference could be driving too fast."

Actually, Kreeger confessed later, "I was doing 40 miles per hour in a 25-mile zone. Then, when I got here, I was turned away by three garages. Maybe they heard about my speeding fine."