The 34-year-old black cap has represented New Zealand in an international career spanning 11 years.
He was released from his contract at New Zealand Cricket in 2012 to focus on the shortened format of the game.
He's appeared 99 times in the t20, scoring 66 as his highest in December 2007.
"I have admired Jacob's all-round qualities for some time now and his t20 experience, which he has gained from global competitions, is a major plus factor in our desire to have a successful Friends Life t20 campaign."
George Chesterson MBE has passed away at the age of 90.
Chesterston represented Worcestershire County Cricket Club between 1950 & 1957, making 47 First Class appearances.
He later served as Club President between 1990 & 1993.
Martyn Price, Chairman of Worcestershire CCC, said: "George's involvement in many aspects of the Club over the years, as a player, President, and latterly his involvement with WOPA means he was well known by many of our members and supporters and he will be remembered fondly by many people."
Saeed Ajmal will be unavailable for Worcestershire Country Cricket Club's Twenty20 campaign because of commitments to his national side.
Ajmal had signed a contract to appear in Worcestershire CCC's t20 run at a time when the Pakistan tour to Sri Lanka was not agreed in the ICC schedule, but it is now understood that it is likely to take place between late May and mid July 2012, ruling Ajmal out of the County's t20 campaign.
Director of Cricket, Steve Rhodes, commented "losing a player of Saeed's quality is a big disappointment, however, we are extremely active in finding a replacement for the t20."