ATK is not a character attribute, it is an attribute of weapons.
STR does influence total atk damage however.
Set a parallel process
Condbranch: Item(MagicSphere) held
then increase attribute:STR by such and such
You want to manipulate attack I suppose, which is the attribute of a weapon.
So,if bronze sword is equipped
and magic sphere is held
equip bronze sword+ (Which is the same thing, but with higher attack.)
This is somewhat inconvenient, but you could pull it off.
Otherwise, shouldn't modifying strength be enough? I'm sure there is a script that modifies attack directly.
What a pain, I couldn't find a condbranch that detects a player attack being used. But if you could, you could use the event command 'deal damage' inside a common event in a monster group. This could add a specific amount of damage if 'magic sphere' is equipped.