When welding armour plate, it is normal to preheat the area. As you are simply using it as filler rather than for bonding purposes, you ought to be ok with just mig welding it. But preheating it wouldn't hurt. Preheating will reduce the chances of warping or cracking.
Perhaps you need to talk with a mat tech from the forces. They get some training on welding armour. But in order to retain the armour strength, I'm pretty sure they would be using arc with special rod.
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