Ever started a hack on FireRed, LeafGreen or Emerald, you got the items completely, edited; but, than, oh crap, the icon's still a "?", well, I'm gonna try to explain how you can fully customize and add more items to the game without, hopefully replacing old items. You can still do all of this on Ruby and Sapphire, it's just all you'd need there is Item Manager and maybe FSF.
Item Image Editor
Advance-Mart/Visualboy Adavance (Maybe, probably just to see if it worked).
And the final result:
It didn't completely work, but you did get a new item out of it!
See, the Dream Mail, which I used to edit the Aura Stone in, is completely unaffected!
If this gets approved and helps people, awesome, I found something that doesn't seem to have a tutorial that some might be interested in.