Assymetric extrusion value entering in both direction?

You don't specify what version of Creo you are using. I am using Creo 4.0

If I create an extrusion, when I've finished defining the sketch that will be extruded, I'm given a ribbon with a number of options (make it a cut, create a surface, etc.

One of the tabs on that ribbon is called "Options". If I select that I have a bunch of methods of doing the extrusion, I can specify depth on what they call Side 1 and Side 2. This is where I would, if I wished, do an asymmetric (in terms of depth) extrusion. Put two different depths for each side, and there you have it.