Combining Existing PDF File with Docbook Output

Is it possible to combine an existing PDF document with a PDF created 
through the docbook toolchain?  I've got a technical manual that is 
being run through the toolchain and output as PDF, but I'd like to 
attach, as an appendix to the manual, some circuit diagrams that are 
already in PDF.  The final result should be a combined PDF file, vice 
linking between the two documents.  I can do this manually with the two 
PDFs, but I would prefer to do it through the toolchain if possible.  
I'm using Xalan and FOP 0.92 beta.

Thanks,
Dave