Typically, you can only add specific item types to each diagram type; however, you can add a Note to a diagram and then link any item to that Note.

Through Stereotypes you can set up an item so that when you drag that item to a diagram, Modeler automatically creates a Note that is linked to the item you dragged. A Stereotype applies this behavior to an item when the Create As Note Symbol check box is selected on the Directly-Stereotyped Options tab of a Stereotype's Property Pages.

After creating a linked Note on a diagram, you can link the Note to other items on the diagram through Note Links.