Well, the problem is quite delicate. First of all because you need to know when the shards hit the units. I came with a relative solution (durations can be modified, that's why I gave no numeric value for it, you can test with different values till you find that they are ok).
Trigger 1 Event - An Unit Starts The Effect of an Ability Conditions - (Ability Being Cast) equal to BlizzChill Actions - set BlizzCast = (Triggering Unit) - set BlizzPoint = (Target Point of Ability Being Cast) - Wait X seconds - Turn on Trigger 2
Trigger 2 Event - Every X seconds Actions - Pick Up Every Unit Within Y from BlizzPoint and do actions -> Create 1 Dummy for (Owner of BlizzCast) -> Add FrostNovaBlizz to (Last Created Unit) -> Add 1.00 seconds Expiration Timer to (last created unit) -> Order (Last Created Unit) to Undead - Lich - Frost Nova (Picked Unit)
Trigger 3 Event - BlizzCast Stops Casting an Ability Conditions - (Ability Being Cast) equal to BlizzChill Actions - Turn Trigger 2 Off - set BlizzCast = No Unit
Now, let's discuss certain stuff. BlizzChill is your blizzard ability you enchance. FrostNovaBlizz is just a Frost Nova ability for the dummy unit which does 0 damage and you set the duration to how long you want the chill to persist.
X is the duration between two waves of the blizzard. Y is the area of blizzard spell. BlizzCast is an unit variable. BlizzPoint is a point variable.