Simmons replaces Dale Sveum, who moves into the third-base coaching box.
The 58 year old Simmons was working as an assistant to San Diego general manager Kevin Towers for the past nine seasons.
The Milwaukee Brewers also officially announced that they had declined the $9-million option for 2008 on the contract of outfielder Geoff Jenkins. The Brewers instead paid a $1-million buyout and made Jenkins a free agent.
Six other Brewers filed for free agency: relievers Francisco Cordero, Scott Linebrink and Ray king, catcher Damian Miller and infielders Tony Graffanino and Corey Koskie.