(In fourth paragraph, corrects to fifth person confirmed)

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 without elaborating.

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.

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

A formal announcement is expected Wednesday. (Reporting by Corbett B. Daly; Editing by Leslie Adler) (For more Economics news and our MacroScope blog, go to blogs.reuters.com/macroscope)