Question on Shockblade spell from Merc Adventures

[Hannibal]

The shockblade spell (lvl 8 combat magic, found on p.16 of the Merc Adventures sourcebook) lists the damage as 1d6 / level of the caster. Does this damage stack on top of the weapon's original damage, or does it replace the blade's original damage?

So, for example, with a 3d6 sword and a level 4 mage, will the weapon do 7d6 or 4d6 damage?

Thanks in advance for the input!

[dragonfett]

I would take it as additional damage due to the fact that on the range entry, it specifically states that it "enchants a physical blade weapon."