The amended complaint, unveiled Friday by attorneys at the nonprofit Bet Tzedek Legal Services in Los Angeles, now includes Walmart Stores Inc.'s name as a defendant. The suit currently includes more than 200 plaintiffs but ultimately could include 1,800 workers.
The motion is scheduled to be heard Jan. 7 in U.S. District Court, Central District of California.
The existing action targets Schneider Logistics Inc., Schneider Logistics Transloading and Distribution Inc. (collectively known as Schneider) as well as Premier Warehousing Ventures LLC, Rogers Premier Unloading Services (collectively known as Premier) and Impact Logistics Inc.
Schneider is subcontracted by Walmart to run the three Eastvale (formerly Mira Loma) warehouses and Premier and Impact serve as staffing agencies, supplying needed workers.
"Walmart's name does not appear on any of these workers' paychecks, but it's become clear that the ultimate liability for workplace violations rests squarely on the shoulders of Walmart," attorney Michael Rubin with Altshuler Berzon LLP in San Francisco said in a Friday morning press conference.
The wage-and-hour lawsuit seeks millions of dollars in restitution for unpaid wages and penalties.
The suit alleges the workers - many of whom are immigrants - were being underpaid, not paid for overtime and in some cases not paid at all for the work they performed.
Bet Tzedek attorney Kevin Kish explains it this way:
"They were being paid on a piece-rate system for each container they unloaded," he said. "Some containers might take two hours to unload while others could take eight hours. They were falsifying hours, saying it was two hours when it might have been three hours - or even eight."
In other cases employees were not paid for additional work they did, such as sweeping floors or labeling merchandise, Kish said.
When workers tried to figure out how they were being paid they were suspended, dismissed or not selected the next morning, he said.
Sometimes Schneider didn't know how much merchandise was coming in to the warehouses until the day the products arrived, Kish said.
"On those days there would be a lottery," he said. "They would say, `We need 25 people today,' but maybe 50 people showed up. So the ones who weren't picked weren't paid at all."
That, according to Kish is against the law. Walmart officials could not be reached Friday but company spokesman Dan Fogelman did respond to the issue in an email response sent to NBCnews.com.
"We disagree with (Rubin's) characterization," Fogelman said. "While we have a set of quality standards that must be met, the third party service providers we utilize are responsible for running their day-to-day business. They manage their people completely independent of us."
The California Labor Commission conducted a raid of the Eastvale warehouses in October 2011, a week before the initial lawsuit was filed, Kish said. They issued citations against Schneider for failure to keep accurate records. The company was subsequently fined millions of dollars.
Officials from Schneider could not be reached Friday.
The Eastvale warehouses provide a critical link in Walmart's supply chain. Merchandise from all over the world comes into the ports of Long Beach and Los Angeles where it is trucked to the warehouses to be unloaded, sorted and re-loaded for shipment to Walmart distribution centers and retail stores.