To add a connected polygon to an existing mesh

1. Select the polygon you want to append to.
2. Select Edit > Append to Polygon Tool.
     The border edges highlight and appear thicker.
3. Click the border edge you want to append to.
    The edge you select is the first edge of the new face. Arrows indicate the edge direction.
4. Click to place a vertex for the new polygon. You can also click another border edge to use it in
     the new polygon.
non-boundary edge?