Four DTD's into one message

I have to create one message, but I do not have one single format, but four DTD's which have to put together.

My scenario in detail: I have to create a RosettaNet-Message for communication with my business partner, but since we have only a few per year, it is not really reasonable to pay the licence for the RosettaNet adapter.

Our business partner provided DTD's for preamble, service header, delivery header and PIP3A4. I implemented this scenario in business connector some years ago, where I 'glued' the DTD's into one string. This works.

Now I want to migrate this interface to XI, but how ?

Regards,

Robert