Looks like you will need GMAX or 3DSMax to sort that out unfortunatly. The german helmet has no collision mesh at all, so you will need a modeling program. HOWEVER. I can provide you with one that does have a collision mesh as I do have the needed program to do this with. It will be attached below my post via forums attachment system. I already gave it mat ID 50 for you. Even though I included the RS file, I think you only need to replace the SM file. Hopes this helps.
GerHelmet.rar
Also, I don't think adding collision mesh to the helmet will stop the bullet from hitting the collision mesh of the soldier's head. I think the collision mesh will need to be enlarged quiet a bit first or something.....