Retirement plan recordkeeper ExpertPlan Inc., of East Windsor Township, near Princeton, last week bought Automated Data Processing Inc.'s ADP Retirement Services unit, Overland Park, Kansas, for an undisclosed sum.

The deal adds about 55 ADP workers to ExpertPlan's previous staff of 70. ExpertPlan now keeps books for 14,000 plans worth $4 billion, up from 8,000 plans worth $2.3 billion before the deal, says vice president Stacey Caserta.

It's the third deal for ExpertPlan since April 2008, when ExpertPlan bought software developer Trust Company Services, also based in East Windsor. In February, ExpertPlan bought third-party benefit plan administrator Actuarial Enterprises, Newtown, Pa., and moved the operation to East Windsor.

Ten-year-old ExpertPlan has been headed by ceo Julian Onorato since 2006. Investors include Argentum Group, New York; Liberty Venture Partenrs, Philadelphia; and Meridian Venture Partners, Bala Cynwyd.