Ohhh, think I finally understand, so they eat some metal and wear others? That way they can fill and then burn metal inside them to fill the metal outside. Also that little bit they fill can charge the stuff inside them....
Hmm it sure would be nice to be a steel compounder... Say, now that we're onto that subject, some would be pretty useless to compound for example:
Copper (how can you compound memories or knowledge?) and ones like aluminum, duralumin and tin wouldn't be that helpful to compound.