Document signed Event notification message

When a Document is signed for a given Signer, in the call back notification, I see following information "name":"DOCUMENT_SIGNED","sessionUser":"SignerID2","packageId":"XXXXXXXXXXXXXXXXXXXXXXXXX","message":null,"documentId":"Document_ID" if we see, message is coming as null. Is it possible to get some information in message? If needed, we can have that information defined in the input json, when package is being created. If we add package/transaction custom data when creating package, is it possible to get that data in notifications?