Kenneth Y. Tomlinson, a Reader's Digest editor and reporter for 14 years, is expected to become the next director of the Voice of America, the Reagan administration's third appointee to that post.

Officials declined to discuss the nomination until it is announced by the White House.

Tomlinson would replace John Hughes, who last month became the chief spokesman for the State Department. Hughes headed the overseas broadcasting agency for only four months, succeeding James B. Conkling, the administration's initial appointee. Conkling resigned in March, saying he was frustrated by disputes and bureaucratic complexities surrounding the post.

The VOA and its parent organization, the United States Information Agency (USIA), have been under pressure from conservatives to be more vigorously pro-American and anti-communist. On the opposite side of that argument, alarm has been expressed that, as an official advisory commission report stated, "more strident programming" could "cast doubt on VOA's commitment to accuracy and objectivity."

That controversy subsided in the months that Hughes was VOA director.

Tomlinson's professional experience, like Hughes', is in writing, not broadcasting. He is 38, a native of Galax, Va., and was a reporter on the Richmond (Va.) Times-Dispatch before he joined the Washington bureau of Reader's Digest in 1968. He reported extensively in this country and abroad for the magazine, and became an associate editor in 1971 and a senior editor in 1976.

In a related development, USIA director Charles Z. Wick said yesterday that he has explored the possibility of extending some VOA programming to the American Forces Radio and Television Service, which broadcasts to U.S. troops overseas.

Wick said he first discussed that in April with Defense Secretary Caspar W. Weinberger, in terms of making joint use of some facilities and programs of the two agencies. Similar proposals, made in earlier administrations, were opposed by the armed forces network on legal grounds and for fear that it would expose troops overseas to political proselytizing.

His purpose, Wick said yesterday, was to determine if audiences abroad could be expanded and costs cut by "combining forces" of the agencies. The armed forces network uses some of VOA's facilities to broadcast its news, special events, public affairs programs and sports events.

However, it is against the law, sources said, to mix programming from the VOA and the armed forces network, and Wick said USIA's exploratory talks with the Defense Department reached that same conclusion.

"Our charter is international," Wick said yesterday, while the armed forces network's "is domestic," and therefore "they are incompatible."

That is, VOA is directed at foreign audiences and is barred by law from broadcasting to the American audience, except with explicit exemption. Armed forces radio and television, by contrast, is limited to broadcasting to American troops overseas the kinds of programs they would receive if they were in the United States.

Agreements with countries where U.S. troops are stationed are based on this distinction, even though some foreign audiences can receive the armed forces broadcasts. News and other programming for the armed forces network generally are taken directly from major American networks, with commercials replaced by announcements of interest to service personnel and their families.

"There is no prohibition against our using armed forces broadcast facilities," or vice versa, Wick said. That more limited combination of resources, including possible joint use of satellite broadcast facilities, will be explored further, but "on a low priority," Wick said.