Hi Matteo,
If the item list is not important this can be achieved using the "User Defined" callout type.
First create a user defined callout on each part, then select the items in the item list and in the Item list ribbon select the "Group" option.
This create a single callout with multiple leaders, then you simply double click the callout to edit the label text and type in the required text.
This does result in a single label with multiple leaders but the item list will reflect the grouped items so this may not be the result you need.
Hope this helps
