You can bring in a question group (.lsg file format) that was previously exported from another LimeSurvey survey.
Importing a question group from a .lsg file is a classic-editor feature. The new editor has no group import: the picker's + Add question group only creates a new empty group, and a group's ... menu offers only Duplicate group and Delete group.
Structure can only be changed while the survey is deactivated — on an active survey the purple + button is disabled.