This works only if any of the connected model elements is missing from diagram: dragging an existing relationship to the diagram adds it.

However, if both model elements are present then there is no way to add an existing relationship to the diagram. This is frustrating, because this is a frequent scenario:
- relationship may have been added through another diagram, now it needs displaying on original diagram
- relationship may have been deliberately removed from diagram (to lessen clutter, etc.), but must be redisplayed after all

current workaround: remove at least one connected model element, then drag relationship to diagram. however, this is a "poor man's" solution, because:
- it will add other relationships (possibly unnecessary on current diagram)
- all formatting, connector routing, etc. is lost, must be done again

this is a continuation from
staruml.uservoice.com/forums/245359-feature-request/suggestions/6073176-need-feature-to-add-relationships-to-diagram-when