I would like to add watermark "not accepted" into the document at the start of acceptance workflow. The watermark should not be removable to the date of acceptance. Any suggestions?
You're gonna need a custom workflow action that uses the OpenXML API to add the watermark to the document.
If your documents are PDFs, people have combined Nintex and Muhimbi's PDF converter to do this in a custom activity - http://murali007.wordpress.com/2010/03/09/adding-a-watermark-to-a-pdf-file-from-a-sharepoint-workflow/