Link agreement to client

Hi.

We have a workflow to which users can attach any custom document.

The workflow enforces the signature path. Making a legal approval mandatory.

What I want to achieve is, that I can download the signed agreement via the API. Then being able to link it to a client.

I assume that the correct way to get the client information would be to add a "signer input field" called "client" and add this information to the document.

But as the users can upload custom documents (and we will not be able to enforce using specific templates) we can't add form fields to the documents.

Even though we created the "signer input field", I don't get the field information via the API, as it hasn't been added to the documents. 

Is there any other way to get the client information into the agreement for retrieval via the API?

Thanks