Ok, so let's start to add stereotype support in Beta Activity Diagram.
With last beta version http://beta.plantuml.net/plantuml.jar you can have:
@startuml
skinparam rectangle {
RoundCorner<<action>> 25
BackgroundColor<<action>> blue
BorderColor<<action>> red
}
skinparam rectangle {
RoundCorner 10
BackgroundColor yellow
BorderColor green
}
rectangle outerAction1 <<action>> {
:innerAction1;
}
rectangle outerAction2 {
:innerAction2;
}
@enduml
Is this what you are looking for ?