There are definitely ways to do that, but the ideal method will depend a bit on the type of documents they're working with.  If they're working with physical documents that are being scanned in, it may be easiest for them to open up the existing document in the repository, and click on the Scanning button in the toolbar.  This will allow them to scan directly into the document itself.  If these are not physical copies, it's still possible to combine them automatically, but things get a little trickier because you'll need some way to identify the two files as being linked.

 

If you're able to ensure that the files have the same name, you can use Quick Fields' Document Class Settings menu to append or prepend pages to the original document.  If you can't guarantee that the documents have the same name, or if you're perhaps adding the documents to the repository via Snapshot, your best bet will be Workflow.  You'll need to include some sort of metadata on the new document that matches the metadata on the original document.  Once it hits the repository, you can have a Workflow that searches Laserfiche for the original file, and then moves the pages from the new file to the old one.

 

Best