Not a suggestion for metals, but if you want to get more out of the Frostgrave barbarians, have you considered kitbashing them with the Frostgrave knights?
The knights sprue has a lot of very 'barbaric' heads on it: horned helmets, moustaches, beards, bare heads and so on. And it also has some hefty two-handed axes. The bodies are too heavily armoured to be run-of-the-mill Dunlendings, but they'd work as leader types.
I suspect you could also harvest some good heads from the crewmen and soldier sprues. Along with the knights, that would give you a lot of options to expand
Also, if you wanted to hint at some of the Dunlendings with Orcish blood, you could try putting Frostgrave gnoll arms on one or two. The barbarian arms are great on the gnolls; vice-versa might give you a slightly unheimlich hint of long-armed ancestors ...