How do I add an existing association/dependency to a diagram for an element that is already on the diagram.

For example, I have an existing class diagram that contains the classes Foo and Bar. There is a association from Foo to Bar that is not displayed in the diagram that I wish to show.

I can see this association in Foo's association list but cannot see any way to add it to the diagram. I do not wish to add all of Foo's associations as I've purposefully removed some of the relations from the diagram.