Do you mean the scan job has all been stored as one or multiple documents? If so, then you need to edit the documents in question and add pages that way. You will need a documetn editor that allows you to scan new pages in and insert them. Adobe tools offer that, but some of them bury the funcitonality so deep in menus that they are impractical. We use a tool called PDF-XChange that does a very good job of allowing pages to be moved around and new pages to be added (it will even convert Word and Excel documents on the fly as you drag and drop them into an existing document -- very handy).
Is this what you need? The bottom line is that once a document is stored to a file cabinet, the scan job it came from is immaterial. You just need to modify the stored document.