Interior Secretary James G. Watt has appointed to the National Park System Advisory Board a 38-year-old unemployed former insurance salesman who has actively opposed government acquisition of property in national parks.

The man, Charles S. Cushman of Sonoma, Calif., is part owner of two cabins in Yosemite National Park and the founder and volunteer executive director of the National Inholders Association, whose 6,000 members also own property in national parks.

He located most of those members from lists he received from the park service in response to Freedom of Information Act requests. In fact, Cushman and the General Accounting Office recently agreed to an installment plan under which Cushman will repay $3,000 to $4,000 the park system charged him to research some of his Freedom of Information Act requests.

Dr. Edgar Wayburn, a San Francisco environmentalist and Carter administration appointee who remains on the 12-member advisory board, said "I don't understand" Cushman's appointment. The board "has been an institution composed of people who were interested in the national parks. Mr. Cushman is interested, but in a way that's different from most of us."

Cushman's views appear to coincide with those of Watt, who has said the park service should concentrate on maintaining what it has and not on expansion.

Cushman is one of six new board members Watt has appointed, but the only one whose name was an automatic red flag to several environmental groups contacted. The other six members are holdovers from the Carter administration. The board, as its name implies, is advisory only, but it can exert substantial influence within both the Interior Department and Congress. Its members receive no salary, only expenses.

William Turnage, executive director of the Wilderness Society, called Cushman's appointment "one of the most stupid and inappropriate things I have heard about this year from Mr. Watt. But I am not surprised. The fox is letting a wolverine into the chicken coop."

Cushman said in an interview that he is not opposed to more parkland. "I'm just trying to get them to say that people are okay. I'm not against acquisition or even condemnation. You've always got some clown who wants to build a eight-story hotel. You've got to stop that guy."

Nevertheless, he said, "We've got to recognize that we're not going to have enough money to buy every piece of land in these national parks. We're going to have to develop partnerships with these people."

Cushman finances his efforts, he said, from investments. He estimates there are 70,000 inholders in national parks and more than 1 million on federally owned land, including national forests and fish and wildlife areas.

Cushman said many inholders tell of being pressured by park officials to sell their land, of being told what color they can paint their homes and of other intrusions on private rights. Cushman said his father was forced to sell property he owned in Yosemite in exchange for keeping his government job. Cushman organized the Inholders Association after some of his neighbors in Yosemite were "subject to heavy acquisition pressures" from the park service, he said