This feature is available in Enterprise and Ultimate editions.
For stories or test sets that can be delivered within a single sprint/iteration, you can add it to a portfolio item anywhere within the Portfolio Tree hierarchy. Note that standard fields (Title, Description, etc.) are copied into the newly-created story.
Select Portfolio > Planning > Portfolio Tree.
Select a portfolio item, and then select an option (Add Child Portfolio Item, Add Child Story, or Add Test) from the Add Child Portfolio Item menu.
Enter the work item details. Note that several key fields are not copied from the portfolio item into the item (e.g., Owners, Priority, and Relationships).
Click Save. The system inserts your new story or test set under the parent portfolio item in the list.
Click the OK & New button to enter multiple child portfolio items or stories to quickly build out your backlog under a portfolio item.