The huge new foundation set up by the late John D. MacArthur, a reputed billionaire insurance tycoon, added seven prominent businessmen, educators and scientists to its board of directors last week.

The six original had been closely associated with MacArthur "in the growth of his fortune," board Chairman Paul D. Doolen said in announing the unanimous election of the new members at a foundation meeting in Northbrook, I11.

But, he said, "Now is the time to broaden the board by adding outstanding experts to help decide the best use of that fortune for humanity."

The new appointees are John E. Corbally, president of the University of Illinois; Gaylord Freeman, former chairman of the First National Bank of Chicago; Murray Gell-Mann, professor of physics at the California Institute of Technology and a 1969 Nobel laureate; Edward H. Levi, former attorney general and president-emeritus of the University of Chicago.

Also, Dr. Jonas S. Salk, president of the Salk Institute and developer of the polio vaccine bearing his name; William E. Simon, former secretary of the Treasury, and Jerome B. Weisner, president of the Massachusetts Institute of Technology.

An expansion of this kind had been sought by MacArthur's son, J. Roderick, who has been pressing for a large, innovative program of grants to enable gifted researchers to pursue their work, in diverse fields, free of the usual red tape and restraints, and who has often been at odds with the othe roriginal directors of the John D. and Catherine T. MacArthur Foundation.

In addition to Roderick MacArthur, the widow and Doolen, who is chairman-emeritus of Bankers Life & Casualty Co. of Chicago, the original directors, reelected Friday, are Paul Harvey, the radio commentator, who has urged funding for a new journalism school and for animal-welfare projects; Robert P. Ewing, chairman and president of the insurance company, and attorney William T. Kirby.

The late billioniare owned outright all of the stock in Bankers Life and left it to the foundation as its sole assest. He died, at 80, last year in Florida, where he lived modestly in Palm Beach and had large land-holdings.

Because the value of tthe stock is still being determined, the foundation hasn't begun to function fully, although it has made grants of $50,000 each to Amnesty International, which seeks to free political prisoners, and the California League of Cities, which is studying the impact of so-called "Proposition 13" tax cuts.

Federal tax laws require a foundation to give away 5 percent of its assets annually. If the Bankers Life stock is valued, as has been estimated, at between $500 million and $700 million, the foundation's yearly donations would have to be between$25 million and $35 million, putting it in the top rank of American phi-anthropic enterprises.

As of late year, the original board was considering the appointment of Mark Cannon, administrative assistant to Chief Justice Warren E. Burger, as president of the foundation and a director. As has been reported, however, that possibility has been on a back burner for months. Doolen's announcement made no mention of Cannon.