Hi Eugen,
The HTTP Load Testing feature is aimed at performing load and performance testing of web applications. Since the feature works by recording and simulating HTTP traffic, it can be used to test other client-server applications that exchange information with the server using the HTTP protocol. I do not have information on whether the HTTP protocol can be used by XBAP applications, but you can try this in case your application can use the HTTP protocol.
An alternative way is to use the Distributed Testing feature. You can find information on it in the
Distributed Testing help topic.