As you won't be able to use them as they are with AmigaOS ATM (I presume you want to use it for AOS, as if you'd want to use it with MorphOS then you'd already have them :-)), I suggest the easiest way to cut them out from a screenshot and use those images.
MorphOS icons are RGBA icons, so they can easily be more than 256 colors and include an alpha channel as well, which is not supported by AmigaOS. (in case of most icons the only drawback will be the jiggy edges, but there may be semi-transparent icons as well which are impossible to reproduce)
You can find screenshots at the morphzone gallery, load them into a draw program, cut out the icon you wanted, reduce the colors, load it to an Amiga icon painter and save it. That's all :-)