It is possible to make a parallel of an ellipse, an ellipse arc, or a spline in a sketch without using a parallel sketch operation. For that it is essential to enable the Solving with profiles option in 2D Sketch > Options (Advanced options).

However, if these entities contain reversal points or if an important modification is made on the spline (important displacement of the tangents) the offset may fail. In this case it is recommended to create an offset operation with 2D Sketch > Operations > Offset.