I think you might be a tad confused as to the ease with which spells are made/changed... Its not a one click process and it really depends on what part of the 'look' of the spell you want to change. Keep in mind that even basic spells have 2/3 different models that create what the "one spell" looks like.
e.g. a fire-ball spell has different parts, there is the the bit that flies to the bad-guy (fire-ball/projectile) and the bit that happens when the spell hits the bad-guy (burning/explosion), etc.]
If you want to change what a spell looks like; you first have to determine what you want your new spell to do. Then, find a base (existing War3) spell you can edit the parts of to make the new spell.
If you change say something like (Mountain King's) 'Storm Bolt' ability into a 'Spear Throw' ability, you will have to change the model for the projectile from the hammer to a spear. However, doing this will just replace the picture of the hammer with a spear. It will still be 'Storm Bolt' everything else (e.g. damage effect, sound effects, target visual effect, etc).
To make a good 'Spear Throw" spell, you would have to tweak all the other stuff like the sound clip, target effect, caster effect etc.
One word of advice, copy the base spell and tweak the copy!
Hope this gets you on the right track!