U.S. Commerce Secretary C. William Verity on Thursday voiced opposition to suggestions that the Soviet Union should be allowed to join an international trade organization. But Verity said the U.S. government's opposition to Soviet participation in the General Agreement on Tariffs and Trade (GATT) could change as the Soviet Union moves toward a market-oriented economy. Some American businessmen supported admitting the Soviets to the group. Verity spoke at a news conference after signing a protocol endorsing joint Soviet-American business ventures. In September 1986, the United States led the opposition to a Soviet effort to join GATT, a 95-nation group that sets international trading rules. ``It would be inappropriate, perhaps impossible, to make any progress if the Soviet Union was a member of GATT,'' Verity said. But he added that ``in time, as the Soviet Union moves towards a market economy, that would be different.'' Verity said Soviet leader Mikhail S. Gorbachev's ``perestroika,'' or restructuring program, is designed ``to move the Soviet economy toward a market economy, and that is important to the Soviet side and to all other nations of the world who would like to see movement in that direction.'' Verity's trip to Moscow comes in advance of President Reagan's five-day visit to the Soviet capital scheduled to begin May 29. Trade frequently comes up during summit talks with Gorbachev, who wants access to American technology and services to help improve his country's ailing economy. Verity and Alexander I. Kachanov, Soviet first deputy minister of foreign economic relations, signed a protocol adding joint ventures to the types of cooperation encouraged by a 1974 trade agreement. Verity's three days of talks with Kachanov coincided with a Moscow meeting of the U.S.-U.S.S.R. Trade and Economic Council, which attracted more than 400 American businessmen. Seven major U.S. companies said Wednesday they formed a consortium to work out business deals with the Soviets. Another company, Honeywell Inc., announced a joint venture on Monday to supply automated production controls for Soviet fertilizer factories. Verity said he stressed to Soviet officials that improvements in trade depended on Soviet easing of emigration restrictions. He said the Soviets had agreed to ``marketing glasnost,'' using the Russian word for openness that characterizes Gorbachev's drive for freer expression.