I'm going to assume you are intending to move skills between characters, like giving Leap to the Paladin or Holy Shock to the Amazon or things along those lines.If that's the case, the post above is correct.
If all you want to do is rearrange the skills or move them between skill tab within a character (like switching Fireball and Lightning Bolt for the sorc), change elemental types, change damage/mana cost, change animations, rename spells, change their levels, assign different icon, etc. ...all of those things can be handled with simple modding, mostly of .txt files.
So no special tools are involved.
But if you do want to do what I think you want to do, you'll need to hex-edit one or more .dll files, and that will require a couple different types of programs. Go check out the Expansion Code Editing forum if that's the case...because for .dll editing issues, the people who hang out there are the people you need. 
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- JEFF
"A hero is no braver than anyone else. He is only brave five minutes longer."
For my latest mod (for LoD/expansion), download Version 1.1 of Sanctuary in Chaos or at this mirror site
- Or for old stuff (for classic D2), visit my orignal mod site at www.planetdiablo.com/sanctuary