I'm not sure just what you mean by 'inserting data to be placed on the pages'.
If you (or your application) is totally in control of writing the PCL6 parent print stream (encapsulating whatever dependent user-defined streams the parent needs), then you should have no difficulty in inserting the appropriate PCL XL operators (preceded by their attribute lists) at the appopriate points in the (highly-structured) print stream.
If, however, you are just using another application (e.g. Word) and attempting to inject occasional PCL XL operators into the output print stream (in the way that you can insert escape sequences for the relatively unstructured PCL5 language) then I don't think that you will get this to work at all.
I don't think that it is possible to set up user-defined streams outside of a parent stream, and then retain (persist) the user-defined streams so that they are available to another parent (job) stream - although I might be wrong here.