First, I have to say Parabolic curves (or any kind of conic curve) can be created directly in a CATIA sketch. Use INSERT+PROFILE+CONIC to get the tools.

But to answer your question: I did a search and found several threads that describe how to make a curve from a math equation. Here are links to two:

This was in Eng-Tips:
CATIA products - Spiral cmd

And this was in COE:
Curve creation using Equations > Active Discussions > COE