Post a document to a 'Written' Agreement using the API

We sometimes have customers who forgot to sign there agreement but still show up at their appointment. Because our current infrastructure does not allow it to fill in the documents digitally we would like to create a new 'Written' agreement that is then signed by the customers and uploaded to Adobe sign. How does this workflow go? Do you need to create a transient document of the written agreement and link that to the new agreement which is then set to the status 'Signed' ? Or do you need to upload the document differently? Our reasoning for this is that we would like all of our agreements to be stored in a single place.