The feature 'Add Related Elements' is available from the context menu. Choose an element in a diagram, right-click on the element and choose the menu item 'Add Related Elements'. If in the model there are defined relations from/to selected element to/from other elements, they will be listed in the sub menu. If there are no related elements, the submenu will be not displayed.
After clicking on submenu item with the name of a related element, the element with the relation to the selected element will be inserted to the active diagram.