If I understood delgado correctly, I think you don't really need the mass here.
What you can do is on collision between the dart and the rock, create a distance joint between the two (not a pin, pin isn't relied to physics, it might end up with funny results). You can also add an impulse to the rock, that is a ratio of the dart's velocity.
From there, I think it would make the rock/dart move as a single object and give the impression that the momentum of the dart was transfered to the rock.