An unusual coalition of processors and workers who turn raw farm products into flours, meals and sweeteners will launch a campaign in New York today to get a bigger piece of the U.S. agricultural export pie.

Their idea is that the conversion of some raw commodities to finished products before they are exported under trade agreements would increase U.S. industrialization and broaden the tax base.

U.S. agricultural exports--mostly raw products--will surpass $40 billion this year. But as overseas sales have risen steadily since 1970, the proportion of processed foods has fallen.

The Export Processing Industry Coalition (EPIC)--a dozen labor organizations and trade groups for corn, flour and soybean processors--will announce plans today for a year-long educational and legislative effort to stimulate overseas sales of processed food products.

The EPIC plan already has the tentative blessing of Agriculture Secretary John R. Block, who acknowledged recently that the administration is weighing the idea of including processed products in farm trade agreements.

This includes a proposal to include processed grain and other products in negotiations on a new long-term agricultural sales agreement with the Soviet Union.

The secretary said that items such as flour, high-protein corn gluten feed, soybean meal, meat and other products could be included. He stressed, however, that the administration has not determined that it would insist on this in talks with the Soviets.

The EPIC proposals to be announced today in New York call for a series of changes in government policy to stimulate the foreign market potential for processed foods.

They will urge Department of Agriculture officials to negotiate bilateral grain agreements requiring that a portion of the sales include processed agricultural goods.

Herbert J. Waters, a Washington agricultural consultant and one of the organizers of the EPIC, said that such a policy, in combination with other steps to open now-closed foreign markets to processed goods, could have a substantial effect on the U.S. economy.

Waters and the EPIC point to a recent USDA study of the economic ripple effect of more exports of processed items--a multibillion-dollar boost to the U.S. economy if only wheat, for example, were included.

The study noted that the United States exported $6.4 billion worth of wheat in 1980, but only $211 million worth of wheat flour. If 10 percent of present wheat exports went abroad as flour, an additional $5.6 billion of business activity would be generated here, with 122,400 more jobs and an increase in personal income of as much as $1.2 billion.

Almost that much more personal income, plus some 160,000 jobs and about $8 billion more in business activity would result if 10 percent of current corn exports was marketed as corn wet-milling products.

William H. Taylor, president of the corn council of the Oil, Chemical and Atomic Workers International Union and a leader in the EPIC, told a Senate subcommittee last month of labor's "serious concern" over the growing export of U.S. agricultural raw products.

"It means we are exporting job opportunities. . . . We are helping create processing jobs in other countries at the expense of lost jobs here," Taylor said.

The EPIC involves a dozen unions, including food-processing and construction organizations and the Industrial Union Department of the AFL-CIO. Industry groups include the soybean processors, the millers' federation and the Corn Refiners Association.