What version of TestComplete are you using? I'm not extremely familiar with XBAP but the little bit I do know is that it is a Windows Presentation Foundation style application and, as far as I know, the recent version of TestComplete (8.50) supports WPF applications as Open applications.
Have you tried recording actions against your application to see how it works?
Basically, TestComplete does not prevent you from working with any kind of application from any kind of framework. Where the difference is comes from whether or not TestComplete sees the application as "open" which exposes native properties and methods to the tool but any application can be exercised through the GUI via TestComplete.
BTW, the most recent version of TestComplete is 8.50.