You would basically want to "pass" the document href to etherpad..
So basically, you want to generate an ID that you store, this is the ID of the pad. You then want to pass the href of the document to the import API endpoint with the ID of the pad included in the request, something like...
Then store the padID with a relation to the documentID in your store. That way the next time you try to access the document instead of importing you will hit the document.
This is all theoretical btw, someone will need to write the ability to pass a href to the import endpoint.