Here's the method I've used for Mattahan's very cool Umicons and GANT sets:
On my PC, I export all the icons as 128x128 PNG's using Axialis IconWorkshop. Then on my Mac, I run those PNG's through Pic2Icon to create resource forks containing them as Mac icons.
After that I've done something you won't have to.. I created a shell scripts that makes the PNG's be empty files, while retaining the resource fork (Mac icon), then I remove the .png extension. I do this because I like being able to browse my icon archive while having the Finder set to column view mode, so that when I highlight an icon, it will show up in the preview column, instead of the contents of the PNG.
Hope this was somewhat clear
