Custom Alert Icons

About a zillion years ago, when I was running OS 8.1…
I used to be able to place a custom alert icon in my dialog boxes using ResEdit.
Is this still possible using OS X?
If it’s doable, please let me know how. I’ve been away from AS for quite awhile, and I didn’t retain much of my previous knowledge.
Thanks in advance for your input.

Well, you can append “with icon #” but replace the “#” sign with either 0, 1, or 2. Or, if you’re using something like xCode, then just make a new .nib file and have it pop up.