As far as I know, you can't unless you want to mess with RAM/ROM addresses, or maybe ASM. There's no way to do it with just Hyrule Magic.
Edit: Grass itself doesn't drop items itself. You insert an Item, (Choose Item in HM, and put one on the grass.) and then you set the value to the item you want (I have no idea how to edit the value of an item, so don't ask me).