Only .PDF and .TIFF can scan multiple pages and compile them into a single document when using the HP software. Everything eise is a flat image format and will create individual files for each page. First of all make sure you are scanning in .PDF or .TIFF before you go any farther. You will know by looking at the file extension (.something) of your scanned documents.
If you are referring to the option to manually add pages to an existing scan job then the HP Software will need to have the scanning preview option enabled. From the scan preview window you can click a button to manually Add/Remove scan jobs to the same document as well as re-orient the pages so they are saved correctly before publishing.