Workflow Designer currently has more or less of the same problems with table fields as AIX; you can read from them but not write back to them easily.
My Workaround for this problem would be to create a Webservice which is called from your workflow - Webservice connnects to SQL Databasase, retrieves all required info and pushes them back to the DW document via .NET API (https://developer.docuware.com/dotNet_CodeExamples/workWithTableFields.html
A not-so-nice and not approved by DocuWare workaround would be to write the info directly from your "great Plains Database" into the DWDATA database via TSQL or something along these lines.
Hope this helps (a bit).
Greetings from Germany,
Simon H. Hellmann
DocuWare System Consultant