How to add a new point with a smooth curve?

[_Aka_]

The code used to draw spline from the editor uses the InsertAfter method, which is the same used in the Add.
If you want to check the editor spline drawing code, you will find it at TBDrawControlPoints.addCP (private method)
If you want to see an example of runtime drawing of splines, you can find one in the script used in example scene 51, InfiniteTrack.addTrackCP (private method)
Let me know if you managed to find what was wrong with your code using the Add() method