You could use Laserfiche Snapshot to print the PDF directly to Laserfiche.  This Laserfiche component acts similar to "print to PDF" with which you may already be familiar and does not require you to physically print the PDF file and rescan it.  Once it is printed to Laserfiche you can merge it with your existing documents as you do now.  To clarify this is somewhat dependent on your user settings in Laserfiche.

PDFs are considered electronic documents and TIFFs are considered images.  Laserfiche has the capability to edit and merge TIFF content together between entries.  It can't directly merge a PDF with existing TIFF content.  If your user settings generate pages when you import a PDF through whatever means then TIFFs are created in addition to the electronic PDF document.  the once you have the pages generated you can move those to the existing TIFF document and decide what you want to do with the remaining electronic document (keep the PDF as a separate electronic document with its pages or TIFF content moved, or delete the electronic document)