I did find a reference to this on Rebecca's Blog. Web AppBuilder Developer Edition - Customization Resource List
Here is the reference... Get around using ArcGIS Online and Portal with WAB
YES YOU CAN DISPLAY YOUR OWN ARCGIS SERVER MAP SERVICES IN WAB
WAB displays web maps that are composed with the ArcGIS Online map viewer. You can compose a web map that includes map services from your own ArcGIS Server. Here is a sample web map that combines a map service from my own ArcGIS Server with the Esri World Topo basemap. The first step in accomplishing this is to register your ArcGIS Server map service with ArcGIS Online: (see snippet from ArcGIS Online Help below)
When you add an item from the web, you are referencing the REST endpoint (URL). The website does not store the item itself. You can reference ArcGIS Server web services, KML, and OGC WMS. You can also reference documents and images stored on the web.
Once you have added your ArcGIS Server map service(s) as items in ArcGIS Online you can add them to your web map. The final step is to reference the web map in WAB.