Click Home tab/Modify panel/Edit Polyline.
(Find)
Select a polyline, spline, line, or arc to edit. If you selected a spline, line, or arc, press Enter to convert the selected object into a polyline.
Enter "J" (Join).
Select one or more polylines, splines, lines, or arcs that are located end to end.
Press Enter to end the command.
Spline-fit polylines return to their original shape when joined. Polylines cannot be joined into a Y shape.
If the properties of several objects being joined into a polyline differ, the resulting polyline inherits the properties of the first object that you selected.