The Justice Department has hired an experienced prosecutor from Baltimore in a move to beef up its staff of lawyers for possible trials leading from its South Korean influence-buying investigation.

Jeffrey S. White, 31, an assistant U.S. attorney in Maryland for the past six years, will join the department's public integrity section about Aug. 1. Justice Department officials said yesterday. He is expected to head the prosecution team for any trials coming out of the Korean investigation.

Benjamin R. Civilette, head of the department's Criminal Division, said in a telephone interview yesterday that White's appointment is an indication that the South Korean investigation is progressing fast enough to require additional manpower.

He emphasized that White's hiring did not mean a charge in direction for the year-long inquiry. Paul Michel, a former member of the Watergate Special Prosecution Force, will remain in charge of the day-to-day investigation, Civiletti said.

Civiletti has refused to speculate about possible indictments in the case. But another Justice Department official said yesterday that some indictments are eqpected and that White will be trying some of the cases."

White is the first of several "super-grade" trial attorneys Civiletti plans to hire to prosecute nationally prominent criminal cases. He will be said about $40,000 a year.

White's appointment focuses attention again on questions raised recently about the pace and scope of the investigation, which centers on allegations that the South Korean government spent hundreds of thousands of dollars a year in cash, gifts and entertainment on members of Congress who supported continued U.S. aid for the regime of President Park Chung Hee.

For example, in recent weeks, Republican congressional leaders have called for the appointment of a Water-gate-type special prosecutor to head the inquiry. And many members of Congress have written Justice expressing concern about problems the investigators have experienced.

Several factors, including difficulty in tracing cash payments, questions about the admissibility of U.S. intelligence reports in court, and missing witnesses, have hampered the inquiry.

Tongsun Park, a Washington businessman who made cash payments to several members of Congress, left the United States for London suddenly last fall, for instance. Since then he has resisted Justice Department attempts to obtain his testimony, even rejecting offers of immunity from prosecution.

In answer to congressional concerns about Park's unavailability, Civiletti wrote several members last month to assure them of the diligence of the investigators.

He noted then that "several hundred" persons had been interviewed and more than 50 called before a federal grand jury here.

He said yesterday that additional personnel have joined the South Korean investigation since he and Attorney General Griffin B. Bell took office earlier this year.

The latest, White, is best known in the Baltimore office for his role in 1975 in prosecuting several people involved in extorting cash kickbacks from subcontractors working on the Calyert Cliffs nuclear power plant in southern Maryland.

Both Civiletti and Russell T.Baker Jr., one of his deputies, were once assistant U.S. attorneys in Baltimore.