(--------)
You can have :
@startuml mainFrame foo GrowersAssociation -> Grower : sendPaperwork Grower -> GrowersAssociation : signContract @enduml
As workaround, you can put an image in a diagram like this:
http://www.plantuml.com/plantuml/uml/SoWkIImgAStDuN8hIinDLL1A104bXMgkejJaaiIyz1oWc4rcRhfLHab9WPM-VdbvkLv1Jc8UI8zUSdwkFeoZNv2NhXzScURcdMLKv5rYx9pldUVg4sJecMyIvP8Vd6Xk00HsIWg9nGg9YLorCFkIaf9eR5jI4j0sBeVKl1IWFW00
@startumlFrame "aaaaa" {rectangle "<img:http://www.plantuml.com/plantuml/png/SyfFKj2rKt3CoKnELR1Io4ZDoSa70000>" as aa}Frame "bbb" {rectangle "<img:http://www.plantuml.com/plantuml/png/SyfFKj2rKt3CoKnELR1Io4ZDoSa70000>" as bb}@enduml