i just fused silver and bronze chromatic mantles together to get it (and yes, they are star metal armors)
Yes, starmetals are fusable, and Gree have now added the ability to fuse like elements - so you do not have to use different elements now (you can fuse two armors together that both have spirit, for instance), but beware, fusion is even more random now - you can fuse armors that have no fire element at all and end up with a fire armor.