Can't do it.
Edgecam can't open NC code, it can only create it via the post processor.
It goes Edgecam->Postprocessor->NC code, but can't go the other way.
Most CAM packages come with a 'backplotter' which allows you to visually see what the NC program is going to do, but for some unfathomable reason Edgecam is the only CAM program that doesn't have one.
To see what the NC code is doing, you'll need a third party backplotting program. I use Predator CNC editor and Mulit DNC CNC editor (which comes with their dnc software) - both are pretty good. Predator do a demo version at
Incidentally, if you've got the right licience on edgecam, you can open native CATIA files instead of Iges.
What you'll need to do is backplot the existing NC code to see what it does, replicate the program in Edgecam, and then post process it to your own machine. (i.e. create your own version of the their NC program)