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WASHINGTON, May 19 (Reuters) - Treasury Secretary Timothy Geithner is beefing up his staff at 1500 Pennsylvania Avenue, bringing Lazard investment banker Jim Millstein on as a top aide, spokesman Andrew Williams said on Tuesday.

Williams did not elaborate.

The Financial Times said Millstein would assist Geithner in "working out" some of the government's existing investments in banks and other financial institutions while structuring new investments to stabilize the financial system.

Former Lazard bankers Steven Rattner and Ronald Bloom are already working on the Obama administration's auto task force, which Ratter heads.

Millstein has been managing director of the Restructuring Group at Lazard since 2000, according to his official firm biography.

The bio says his responsibilities at Lazard included representation of the United Auto Workers in connection with its negotiations with GM, Delphi, Ford and Chrysler.

The Senate confirmed Neal Wolin as deputy Treasury secretary on Monday, making him the fifth person confirmed by the Senate for the Treasury.

A formal announcement on Millstein's appointment is expected on Wednesday. (Reporting by Corbett B. Daly; Editing by Dan Grebler ) (For more Economics news and our MacroScope blog, go to blogs.reuters.com/macroscope)