Ehm...
DOpus 5.x has a function that adds an icon to a file or dir via the default icons in ENVARC:
It's called "AddIcon"
And as far as I remember there is a tool supplied with NewIcons that copies eg. A icon of your choice to all dirs... But this only the NewIcon picture informations which is copied... So If you don't use NewIcons this is irrelivent <-- This is not the spelling of that word, is it? :-)
-LP