Is there a printing to PDF application that lets you append to the end of the already existing file?
I'm currently using CutePDF (mentioning this because I may have overlooked such option), it doesn't seem to have such option.
At least PDFCreator is able to combine many printed documents into one, but to get it append to an existing PDF I think you'd need to reprint the original one first (to get it in the print queue in PDFCreator) and then print the docs to be appended with it. You'd probably want to do it this way.
Another way of combining PDF (and PostScript) docs is to use GhostScript suite (I'm thinking of psmerge here in particular) which essentially means converting PDF to PS to do the combining and then converting back to PDF, which probably can mess up some things if the original PDF is of too high version (being closed-source format the support for newer PDF features is slow to appear in open-source tools). I doubt there is any GUI for that but psutils package should contain some sort of psmerge implementation. With this approach existing PDF files could be used directly.