Hargreaves will be responsible for overseeing the development of the club's young players.
The 40-year-old moves to Dean Court after a coaching spell at Exeter.
He told the club's official website: "I'm really looking forward to it. There are some exciting players here and it is an exciting time.
"It can't be underestimated how important it is to the club and area to produce homegrown players. If the player gets into the team and you sell him for a lot of money it's fantastic for the club.
"Long term you hope those players stay at the club. But being realistic you hope they get in the team, you improve them and they go on again."