Plaintiffs in a revived stockholder class action against Six Flags Entertainment Corp. are asking a federal district judge in Texas to allow a Florida-based pension fund to enter the class—a move the defense views as counter to its motion to dismiss.

The plaintiff response is the latest maneuver in a more than 4-year-long battle with Six Flags and its chief executives over allegations the company misrepresented its financial position in relation to three planned amusement parks in China that never got built.