Former Treasury Secretary William E. Simon has joined the consulting firm of Booz-Allen & Hamilton in an unusual "of counsel" capacity.

That means Simon will act as "a senior advisor to the firm and its clients on economic and financial matters" on a parttime basis, devoting one to two days a week to this activity and maintaining an office at Booz-Allen's New York headquarters, but free to spend time on a large number of other duties.

Last week it was announced that Simon - Treasury Secretary under Presidents Nixon and Ford who earned a reputation as a strict fiscal disciplinarian by cracking the whip over both New York City and Great Britain in return for approving financial aid - also will become president of the John Olin Foundation, an organization devoted to advancing the free enterprise system.

Simon, in a telephone interview, said the foundation presidency will be "the centerpiece of my activities," which besides his connection with Booz-Allen also will include some independent consulting work, and several corporate directorships which he has yet to decide on.

"I am going to be very selective as far as going on a couple of boards," he said, adding he did not want to "get myself overcommitted at this point."

Simon already is on the boards of Georgetown University and Lafayette College, his alma mater, and also is treasurer of the U.S. Olympics Committee.

The former Treasury Secretary said his old Wall Street Firm of Salomon Brothers, where he was a partner before he went to Washington and where he reportedly earned several million dollars a year as a bond dealer, had offered him a partnership if he would return. But after "agonizing," Simon said he decided to "branch out."

Why? "Probably because I'm a different fellow than the one that came to Washington four years ago," he replied.