You can use the Fit Curves command in NX to create a quadric curve or a spline by fitting it to Scanned Data.
You can fit splines, lines, circles, and ellipses. The splines, circles and ellipses, can be open or closed.
The key benefit is that this is all feature-based geometry, and it is fully integrated into the NX ecosystem.
In this NX Quick Tips video, Tod Parrella shows us how to use the Fit Curves command in NX 9.