First you need at least the alpha that introduce this new property then you simply edit magic.cfg and add it to Properties field of shot of your choice.
Code:
[shot13]
Name = SHOT_HAILSTORM
Animation = 908
AnimationSize = 96
AnimationTransparency = 2
Size_XY = 48
Size_Z = 48
Health = 40
Damage = 5
DamageType =
HitType = 2
AreaDamage = 0 0 0
SpellEffect = 0
Speed = 220
BaseExperienceGain = 256
PushOnHit = 1
DestroyOnHit = 1
BounceAngle = 0
TargetHitstopTurns = 6
FiringSound = 46
ShotSound = 0
ShotSoundPriority = 1
HitCreatureSound = 137 1
HitWallEffect = 22
HitDoorEffect = 22
HitWaterEffect = 19
HitWaterSound = 36 1
Properties = PENETRATING
Hailstorm doesn't have any Properties by defaut but you can add it at the end of the shot block, like I did here.