XInclude and general entities

Hi,
I don't know if this is even possible, but:
I have an read-only XML file (report.xml) with a DTD that just defines
some entities. I want to add some entities to this DTD but I can't
change the file.. the solution I'm trying at the moment is to build
another file with my own DTD, then XInclude the original file. I
thought it might merge the DTD's but it doesn't seem to work. Is there
any way to accomplish this, or does anyone have an alternative
solution?
Thanks,
Konrad