It's easy to change the type with ProcessIcon:
ProcessIcon
YourIconFile SIT DISK
That works at least on AOS with my icon.library, even for OS4 and PNG TrueColor icons, although ProcessIcon doesn't know these formats and can handle only OS 3.5 ColorIcons. My library is cheating a bit.
But don't expect the Aros icon.library to do the same tricks behind the back of ProcessIcon or any other icon tools. There, it will probably just work with OS 3.5 ColorIcons.