Hi Jumbik,
True, in References -> Stereotypes there is no option for use cases in "Applies to"; true, also, that UML allows for stereotyping of any classifier and any association. But use cases are classifiers (for those not familiar with the concept, these are, basically, UML elements that have both attributes and behaviour), and there is an option for "Applies to" "classifier" in References -> Stereotypes, and you can create the <<external>> stereotype for your use cases there.
After you create this stereotype for classifiers, it will be available in your use case dialogs.
To automatically give these use cases a clear visual cue, just follow the instructions on metafiles in help.
Hope this works for you, and good luck!
Jaime Gonzalez