Sure you can do it. Here is a link that explains how its done.
http://support.apple.com/kb/HT2493
You will need to remove Entourage from the dock, change the icon for Entourage and then put it back in the dock. Just so you know, every time there is an office update it will reset the icon back to the default terrible e, so you will have to change it over again.
If you have Snow Leopard, you may have to change the permissions on Entourage before it will accept the new icon. When you're in the Get Info for Entourage, click on the little lock at the bottom right and change the permissions to read and write and the icon change should take.
There are lots of nice icons out there, DeviantArt has tons of them
http://www.deviantart.com/#catpath=customization/icons/os&order=9
Good luck