Hello Michael Weingand,
Web forms do not have any fields which can be filled, you need to use merge forms for that.
But stamps can be placed on any document, as long as it is not a data record.
Workflow can stamp a document automatically in recent versions using the WebService activity with the Platform API (built-in).
Hope this helps.
Greetings from Germany,
Simon H. Hellmann
DocuWare System Consultant