You can add pages to a note that has already been scanned using the 'Edit' function. Only pages from different notes filed in the same folder can be combined.
Open the note you want to add pages to and tap the Edit icon in the bottom left hand corner. Tap the 'Import pages' icon in the middle of the bottom menu bar (Android) or bottom right (iOS).
Select the pages you want to add by tapping each page. You can chose to copy (left icon) or move (right icon) the pages. If you choose to move all the pages of a note to a different note, the original note will disappear.