Hello,
Some ideas on how this might be accomplished:
I believe you can utilize the DocuSign Document smartobject to go out to Docusign and get the signed document from an enevelope. There is a load method that you can specifiy the Enevlope ID and ID of the document in the envelope (starts at 1).
This will allow you to get the document, but there is no flag/filter to confirm that the returned document has been signed.
Should be able to combine the output of the above to return the file within an upload attachment call to SharePoint/SQL.