NEW YORK (AP) _ Imelda Marcos, facing a welter of legal problems despite her acquittal on federal fraud charges, hired her former co-defendant's attorney Monday to represent her in future court proceedings.

James P. Linn, who successfully defended Saudi Arabian financier Adnan Khashoggi in the case that ended July 2, said he agreed to take over ''all legal affairs'' of the former Philippine first lady.

In a telephone interview, Mrs. Marcos said she was urged to hire Linn by Gerry Spence, the Wyoming attorney who represented her in the fraud trial, earning a $5 million fee.

The Justice Department has ordered Mrs. Marcos to testify Sept. 25 before a federal grand jury in Pittsburgh investigating a nuclear power plant built in the Philippines by the Westinghouse Electric Co.

The plant was built in the early 1980s when Mrs. Marcos' late husband, Ferdinand Marcos, was president, but was never operated because of alleged safety concerns.

It is also the subject of a civil suit against Westinghouse by the present Philippine government, which claims the Marcoses accepted kickbacks in exchange for allowing construction of the plant.

Mrs. Marcos said over the weekend that she had no knowledge of the case and no papers concerning the plant. Linn said Mrs. Marcos would testify unless the government advised her that she is a target of the criminal investigation.

Mrs. Marcos also is involved in a number of civil cases in which the government of President Corazon Aquino seeks to recover millions of dollars allegedly stolen by Marcos during his 20 years in power.

In the fraud case, the federal court jury rejected the prosecution's claim that Mrs. Marcos participated in her husband's financial dealings, or that Khashoggi had falsified documents to help the Marcoses conceal their holdings in New York real estate and artworks.

Linn, a former West Texas prosecutor who now lives in Oklahoma City, led Khashoggi's defense team and will command a multimillion-dollar fee for representing Mrs. Marcos.

Like Spence's fee, the costs will be paid by tobacco heiress Doris Duke, a friend of Mrs. Marcos, Linn said. Neither Linn nor Mrs. Marcos would divulge the amount.

She said the change in lawyers had nothing to do with the controversy that swirled around Spence, who triggered numerous rebukes from U.S. District Judge John F. Keenan during the trial.

''Mr. Spence had to go back to Wyoming where he has three criminal cases, and he suggested Mr. Linn to take over all our cases,'' Mrs. Marcos said. ''Mr. Spence was a good lawyer, and it takes a lawyer to know another good one.''

In another development, the Aquino goverment, which has barred Mrs. Marcos' return, apparently wants to keep her half-sister, a Roman Catholic nun, out of the country as well.

In a cable filed July 11, the Philippine foreign ministry asked the Philippine mission to the United Nations to add Dulce Romualdez, known in her religious order as Sister Bellarmine, to a ''hold-order list.''

If carried out, the instruction would mean that Sister Bellarmine could not obtain a visa to return to the Philippines.

Spokesmen for the Philippine consulate here said they hadn't seen the order.

Sister Bellarmine was among the coterie of 50 or so Marcos supporters who attended the 10-week trial. She has lived with Mrs. Marcos since shortly before Ferdinand Marcos died last September.