This is certainly possible, although this is not what is implemented, as you know, in DocumentBuilder04.
Basically, what you would need to do is to use the approach detailed in Advanced use of DocumentBuilder screen-cast, #6 in the following list:
DocumentBuilder Developer Center/
Instead of simply creating sources, you want to add the special element inside the content control, and specify the source in that element.
As a first step, experiment with the advanced use of DocumentBuilder by writing example programs, until you understand exactly how it works.
-Eric