This feature is available in Enterprise and Ultimate editions.
For large stories that cannot be completed in a single sprint/iteration, you can convert (or generate) the story to a parent portfolio item. Instead of decomposing a high-level portfolio item, you can start with a weighty story and generate a parent portfolio item for it, under which you can begin to break out the work into smaller, more manageable child stories.
All fields, tasks, test set details, estimates, and tracked time remain on the child stories. Standard fields (Title, Project, Description) are copied to the new parent portfolio item to give you a starting point.
Note that the following relationships are elevated to the newly-created parent portfolio item from the child story, allowing peer stories created later to share the relationships: Links, Attachments, Issues, Blocking Issues, and Ideas. Conversations and other related information remain linked to the child item exclusively.
In any grid, select the story from which you want to create a portfolio item, and then select Edit > Generate Parent Portfolio Item.
Enter the new parent portfolio item attributes, and then click Save.