The No. 41 Dallara was originally assigned to Foyt's son, Larry Foyt, who instead will focus on his job as team director.
The 31-year-old Simmons, whose best finish in three previous starts at Indianapolis was 11th last year, will drive the car for the first time Wednesday when practice resumes for the May 25 race. His teammate is veteran Darren Manning.
Simmons drove for Foyt in the developmental Indy Lights Series in 2004, finishing second in the Freedom 100 at Indianapolis. He moved up to the IndyCar Series the next year and drove the past two years with Rahal Letterman Racing. His best career finish was sixth last year at Texas Motor Speedway.