For regular actions, Hive allows converting actions to subactions and vice versa via Action Card → More (⋯) → “Convert to (sub)action”, and creating subactions by dragging an action under another action in Table and Gantt views.

For recurring actions, this is not possible: they cannot be dragged and dropped to become subactions / moved underneath another action in Table or Gantt view, and the same conversion or reordering does not work via the Action Card.