The Reagan administration late last night added an Italian engineering firm to the blacklist of companies defying the United States' trade embargo against the Siberian natural gas pipeline to Western Europe.

The Commerce Department acted after a Soviet freighter left Leghorn, Italy, carrying two U.S.-designed gas turbines made by Nuovo Pignone Engineering Co., Commerce spokesman B. Jay Cooper said.

The sanctions, in the form of a temporary denial order, are designed to "prohibit the export of all U.S. oil and gas equipment, services and technology" to the state-owned company and its subsidiary, INSO, according to a statement issued in the name of Commerce Secretary Malcolm Baldrige.

The terms of the latest order are more limited in scope than those previously issued against Dresser France and the French government-owned Creusot-Loire, but in keeping with what the administration has decided to impose on John Brown Engineering Ltd. of Britain when six turbines it produced that include U.S. components are shipped to the Soviet Union.

"The Commerce Department intends to file motions to modify the temporary denial orders against the French firms to conform to the temporary denial order issued against Nuovo Pignone," Cooper said.

The administration imposed sanctions against the French companies after Dresser France, a subsidiary of Dresser Industries of Dallas, shipped three natural gas compressors to the Soviet Union for use on the pipeline. Dresser France and Creusot-Loire were prohibited from importing U.S. goods, services or technology until futher notice. Senior officials have said the British firm would be blacklisted only from importing U.S. oil and gas equipment and technology.

Baldrige indicated the administration is not backing away from enforcing the embargo that President Reagan ordered following imposition of martial law in Poland.

"Narrowing the scope of the temporary denial orders, on the basis of facts determined thus far in our investigations, fully maintains the goals laid out by the president regarding the oil and gas sanctions imposed on the Soviets without unduly harming our allies and friends," Baldrige said.

"The department will take action against other companiles if violations of the department's export regulations are suspected . . . . "