A Swiss lawyer accused of helping American clients conceal millions of dollars in offshore accounts pleaded guilty to conspiracy to commit tax fraud in federal court in New York. The lawyer, Edgar Paltzer, 57, a dual United States-Swiss citizen, admitted he opened bank accounts in Switzerland in the name of entities he formed for American citizens, knowing they intended to evade taxes. “I was aware that this conduct was wrong,” he said, pleading guilty to newly filed criminal charges as part of a plea agreement. As part of his plea, Mr. Paltzer agreed to forfeit any fees he earned and cooperate with the United States government. “His cooperation is complete and without any limitation,” Thomas Ostrander, Mr. Paltzer’s lawyer, said after the hearing. Mr. Paltzer, a former partner of the Swiss law firm Niederer Kraft & Frey who is licensed to practice in New York, was first charged in April on one count of conspiracy. That indictment accused him of conspiring with American taxpayers from 2000 through at least 2012 to help them hide their Swiss accounts from the Internal Revenue Service.