It looks like you'd have to approach this in a couple of ways.
First of all, I'd consider the pilum a one shot weapon, so there needs to be an appropriate multiplier to reduce the cost of the attack. I'd use something on the order of 0.33.
Secondly, the pilum can affect a target's shield if it imbeds in it. However, it's only going to affect the shield if it "hits" it, which could also very well be considered a miss, in that the target isn't wounded (which in game terms is because the armor and shield die roll is equal to or greater than the attacking die roll).
I think where I'd start is the following:
Pilum:
One shot weapon, with a range band of 1
Special ability of Shieldbreaker.
If a target is attacked and NOT wounded, then the target must roll a d12. On a roll of 7 or greater, the pilum has destroyed the shield.
Weapon cost multiplier (0.50)
My logic here is that with ARES, I've always assumed that a successful attack roll has bypassed any model's weapon skill, and armor or shield effects. Therefore, I'm envisioning the pilum sometimes affecting the target if the defenses in place "work."
The wepaon multiplier reflects the one shot cost (0.33), with the added bonus of affecting a shield 50% of the time, which brings its cost multiplier back "up" to (0.50).
There are certainly other ways in which this could be handled, but I think this is where I'd start.
Kevin