Is it possible to use/load an existing PDF, with som pre-printed elements, as a template, instead of generating a new pdf?

Thnx

No, not yet. We hope to add this one day, but it is a big job.

Is this question referring to dropping a PDF behind the incoming markup as part of the template? This is not possible? Must everything in the PDF come in from the HTML file?

Yes, HTML or from SVG. Some PDF files can be converted to SVG with third-party tools.