Just modify the spell "Boon of Kargan Tor":
Boon of Umath (Rune(Storm), Summon)
2 Points
Touch, Passive, Temporal
This spell must be cast on a weapon. At spell strength 1, it adds +1D3 to the damage done by that weapon for the duration of the spell. If the spell’s strength is increased to 4, it adds +1D6 damage. For each additional 4 levels of strength, it does an additional +1D6 points of damage.
YGWV, but I would say no. If they could do that, it would certainly not be a sorcery spell but rather a Hero Ability of some sort.