Compare the voltage of the iPod adapter with the voltage Creative uses for the Zen. That should be in the documentation.

I just checked my sons. He has a Sony adapter (4.5 V) connected to a Zen docking station that says it expects 5V without any problem. But, of course, you shouldn't use the 19V of a typical laptop adapter!
If the iPod adapter doesn't give more than 5V, it's safe to use.