If you already have existing product backlog items, you can automatically add them to your story map in the appropriate cell. Where the item belongs on the map is determined by its parent work item (entered in the user activity) and its iteration.
If work items can be automatically assigned to the map, an exclamation mark is displayed in the corresponding user activity and/or slice. Click on this icon and select Insert missing work items to automatically assign matchine all work items to the story map.
If items have only been assigned to an iteration, but are not mapped to a user activity, a new column is added for these stories, called "To Prioritize". You can then drag the stories from this column to the appropriate user activities.
Note: This option takes into account the currently selected Query. Only work items returned by the currently selected query are added to the map.