You can add pages to a document in OnBase using the Document Separation tool. 

1. Import or Retrieve the document that you want to add pages.
2. Right-click on the document (or if Retrieved, right click on the document name)
3. Click Send to > Document Separation.
   
4. To add pages from another document on your computer click the Browse button in the Document Separation tab. 
   To scan in hardcopy pages click Acquire.
   
5. The new pages will be grouped below your current document.  Drag the new pages into the current document's group. Or check off each group of pages and click Join > Merge Documents.
   
6. Click Save and Close when finished.  
   Note:  You may not be able to move away from this screen unless all documents are delete or indexed.  

Learn more about Document Separation:  https://youtu.be/X_tBa-ycYXY