I am not aware of a way for the scanner on any HP printer model to process jobs from both the feeder and the glass at the same time. You are more than likely going to have to find a way to load up the pages in one or the other if you want a single continuous document to get created.
The best solution I can think of would be to perform a simple copy of the document on the glass so that you eliminate the sticky residue from teh glue. Then load your copied image into the document feeder to process it as a part of a normal multi page document.
Otherwise if you can get the documents scanned and back to your computer I'm sure there is software out there that will let you modify the documents together. For example with PDFs there is likely a product out there that will allow you to open and merge the documents together to form a new single document before republishing as a new PDF. Tiff likely has similar products but I do not have any experience with either.