Sproles was drafted out of Kansas State in the fourth round. He set the school career rushing record with 4,979 yards, which ranks 11th in the NCAA career history.
The Chargers have three unsigned draft picks, including both first-rounders, outside linebacker Shawne Merriman and defensive tackle Luis Castillo. Also unsigned is wide receiver Vincent Jackson, a second-rounder.
Chargers rookies are scheduled to report Monday, with their first practice on Tuesday morning.