Merge
The Merge tool lets you join elements of different types, such as splines to arcs.
Select the elements you want to join, then click on the Merge tool.
If there is a small gap between elements, you can set the Merge Distance to fill that gap. Tolerance sets the precision you want to hold for the resulting element. If Smoothing is on, the result will be a spline. If off, the result will be a polyline.
Click the Merge button to accept.
Note: If any of the selected elements have Seams, Notches or Anchors the resulting element will not have them anymore.