OVERLAND PARK — A mail-service drug prescription company in Overland Park has announced an expansion that will include adding 620 jobs to its current round of hiring, making the firm Johnson County’s fifth largest private employer.

OptumRx is expected to have about 2,100 employees at its mail and customer service center in 18 months, according to The Kansas City Star. After the expansion, the only private companies in the county that will be larger than Optum are Sprint, Garmin, Shawnee Mission Medical Center and Black & Veatch.

OptumRx has handled 48 million prescriptions since it opened its Overland Park facility in 2006. The 313,000-square-foot site was expected to process 13 million prescriptions this year. Company officials said about 450 of the new positions have been filled or are still available this year, and an additional 170 employees will be hired by 2013.

“We are adding employees in virtually every position, from entry-level to highly experienced, from shipper to pharmacist,” said Larry C. Renfro, CEO of Optum, the parent company of OptumRx.

Company officials announced the expansion Wednesday, emphasizing that entry-level positions, which start at $21,000 annually, require no previous experience in the pharmaceutical industry. The facility currently employs 200 pharmacists who earn salaries of more than $100,000 per year.

The industry referred to as pharmacy benefits manager is poised for continued growth, officials with the firm say. OptumRx, which is part of UnitedHealth Group, has 13 million customers and expects to fill $23 billion worth of prescriptions this year. Besides Overland Park, the firm’s other large mail service center is in Carlsbad, Calif.

“As the population ages and more of those individuals without insurance gain access to health care coverage, the industry will only grow,” Renfro said. “And OptumRx is well-positioned to take advantage of that growth, with the help of our employees and partners here in Kansas.”