i make my disabled icons in a pretty easy way using photoshop, this is how
first u make a custom non-passive icon, save that as BTNname.tga, then, select the image (not border) with the rectangular marquee tool, then press ctrl+shift+i (this will select inverse), then go to edit>fill: black, deselect the border, use the gradient tool to fade out the edges a bit, then press ctrl+u (hue/saturation) then put the lightness to -50%, finally save as DISBTNname.tga, convert it to .blp and VOILA! u have a disabled icon
for a passive icon, do all the same, but dont change the lightness, and save it as PASBTNname.tga but then u need to make a DISPASBTNname.tga
if u have photoshop, use do this method, if not... well... i dunno hope this wasnt too complicated
(some hotkey combinations may be dodgy, i use a mac, so with a PC you have to replace the "apple/comand" button with ctrl)
i do it my way cos i use a mac and the w3maputils dont work, and my way is more reliable (also it gives you the satisfaction of having made the whole icon yourself)