You can't create dynamic stamps in Reader because you can't add the form fields and the scripts required for doing so.
You can use a dynamic stamp in Reader, but you need to have Acrobat to be able to create it.
I guess I should have rephrased my question.
I understood the question. You can create stamps in Reader, but not dynamic ones. For that you would need Acrobat.
I guess other PDF editors can do it too, but I have no experience doing it in another application and I think it will be quite difficult.