Norton Rose Fulbright can be added to a $100m gender bias lawsuit filed last year against its merger partner, Chadbourne & Parke, a federal magistrate judge has ruled.

Former partner Kerrie Campbell (pictured) filed the suit last August, alleging Chadbourne consistently underpaid women.

The plaintiffs attorney, David Sanford of Sanford Heisler Sharp, asked to add Norton Rose to the suit as a defendant on 30 June – the same day the firms confirmed their tie-up.

Sanford also asked to add allegations about a Chadbourne lawyer who sought to join the suit, Mary Yelenick, and add allegations about "material developments" since the complaint was filed.

Chadbourne's lawyer, Proskauer Rose partner Kathleen McKenna, objected to any amendment.

In a discovery scheduling order, Southern District Magistrate Judge Barbara Moses granted Sanford's request to amend the suit "for the limited purpose" of adding Norton Rose and allegations about Yelenick. She wrote that those amendments would simply "conform the pleadings to the current cast of characters," before any substantive motions.

However, Moses denied Sanford's request to add more details about "material developments," reminding the parties of the June order from the Southern District judge overseeing the case, J Paul Oetken, who rejected a proposed amended complaint for more retaliation allegations.

Oetken had denied competing motions and ordered "limited" discovery to determine if three former Chadbourne partners deserved protection under employment laws, or if their former status as partial owners of the firm disqualified them from bringing the suit.

Moses, in her order, outlined 2017 and early 2018 deadlines for discovery and set a 20 October status conference.