You can do the 'ranged splashing damage trick' with barrage ability
But beware, barrage ability will add +2 additional target to the unit
So the unit which has the custom barrage ability can attack 3 unit at once
Oh, and to customize the damage dealt (additional target damage) you can edit it in the unit's data, check for maximal damage, minimal damage, and medium damage
I highly suggest you to use this settings:
Unit:
Max damage: 9999 (as max as possible)
Med damage: to your liking
Min damage: 1
Ability barrage:
Max damage: as max as possible
Min damage: 1
Don't go below 1 or the unit will deal no damage