I haven't explored using the editor widget but here is a code sample that will merge polyline graphics into one geometry:
function mergeLines(graphicsArray){
// merge geometries of graphics into one geometry
var polyline = new Polyline(map.spatialReference);
for (var i = 0; i < graphics.length; i++){
var paths = graphics.geometry.paths;
for (var j = 0; j < paths.length; j++){ //needed for multi part lines
polyline.addPath(paths);
}
}
return polyline;
}
var mergedGraphicGeometry = mergeLines();
the graphicsArray could be map.graphics.graphics, featureLayer.graphics, featureLayer.getSelectedFeatures(), a manually populate array, etc.
Hope that helps!