Unfortunately, there is no out-of-the box auto-complete polygon in the current version of ArcGIS for Windows Mobile. The GPS streaming method used when collecting polygons may accomplish the same thing, if I understood your question correctly. If not, then I recommend your add your comments to ArcGIS Ideas site > Split (join) sketch by line which has an attached user comment asking for the autocomplete polygon functionality. ArcGIS Ideas is a site where users can contribute ideas to Esri and have the community discuss and vote on those ideas.
