Using Other Character's Special Moves:

To use another character's special moves, instead of touching the A button on the Character Select screen, touch the START button on the touch screen. This will allow you to use another character's special move.

Example: You want to play as Itachi, but you also want to use Kakashi's Lightning Blade. What you do is touch START on the Character Select screen, then choose Kakashi and click on his Lightning Blade attack. Place it in one of the empty slots and you're done! This is pretty much automatic for the Third Hokage's Admantine Nyoi.