Merge Existing PDF into new ReportLab PDF via flowables

I have a reportlab SimpleDocTemplate and returning it as a dynamic PDF. I am generating it's content based on some Django model metadata. Here's my template setup:

buff = StringIO()
doc = SimpleDocTemplate(buff, pagesize=letter,
                        rightMargin=72,leftMargin=72,
                        topMargin=72,bottomMargin=18)
Story = []

I can easily add textual metadata from the Entry model into the Story list to be built later:

    ptext = '<font size=20>%s</font>' % entry.title.title()
    paragraph = Paragraph(ptext, custom_styles["Custom"])
    Story.append(paragraph)

And then generate the PDF to be returned in the response by calling build on the SimpleDocTemplate:

doc.build(Story, onFirstPage=entry_page_template, onLaterPages=entry_page_template)

pdf = buff.getvalue()
resp = HttpResponse(mimetype='application/x-download')    
resp['Content-Disposition'] = 'attachment;filename=logbook.pdf'
resp.write(pdf)
return resp

One metadata field on the model is a file attachment. When those file attachments are PDFs, I'd like to merge them into the Story that I am generating; IE meaning a PDF of reportlab "flowable" type.

I'm attempting to do so using pdfrw, but haven't had any luck. Ideally I'd love to just call:

from pdfrw import PdfReader
pdf = pPdfReader(entry.document.file.path)
Story.append(pdf)

and append the pdf to the existing Story list to be included in the generation of the final document, as noted above.

Anyone have any ideas? I tried something similar using pagexobj to create the pdf, trying to follow this example:

http://code.google.com/p/pdfrw/source/browse/trunk/examples/rl1/subset.py