from docx import *
document = opendocx("document.doc")
body = document.xpath('/w:document/w:body', namespaces=nsprefixes)[0]
body.append(paragraph('Appending this.'))
The second line may need to chance depending on where in the file you are going to append the text. To finish this, you will need to use the savedocx() function, and there is an example of its usage in the root of the project.