Simply unpack spells.txt (lacated in ...King's Bounty. The Legend\sessions\base\ses.kfs) edit it with notepad (or any text editor). Find "slow" and there is following:
unit_count=one, one, one
change it to:
unit_count=all,all,all
and you have mass slow from 1st rank (or you can change to one, one, all and mass slow will be at 3rd rank). Don't be surprised, that mass efect is not in description of spell, but trust me it will be mass
.
Similar changes can be made for every spell which require to choose targer (blind, haste etc.).