Link Workflow Document to BKPF object

I have a workflow document object (WF_DOC) that I created in my workflow that I want to attach/relate/link to a accounting document object (BKPF).

Has anyone tried this?

Standard workflow (SAP ver 4.6b) works great and attaches the WF_DOC to the task but I cannot find a way to create a link to it from the BKPF object.

I have tried calling the method CREATE_RELATION from the BDN Navigator class CL_BDS_DOCUMENT_SET but all I get is a 'Nothing found' message.