Tieflings are actually much easier to introduce to the Warhammer setting then Warforged. They're mutants, Chaos touched. Just refluff them to not necessarily have horns and such but random mutations.
Warforged can work, they can be something from:
1) Dwarves, covered with runes.
2) The Engineer College, possibly a creation of an engineer who draws upon the Winds of Chamon, the Lore of Metal.
3) An Alchemitical rogue creation of the Chaos Dwarves.
4) Creation of Necromancy, trying to meld bone with metal.
5) Coincidental. All things are possible in Chaos, even things contradictory to its nature. A Chaos touched creation could stumble from the north, and be bestowed with the same amount of will as any other living creature.
You could also let your player decide which one.
Although I don't see what one of your players being a Artificer has to do with Warforged.