How could a humanoid species evolve to have six arms, all in relatively the same part of the body, the top corners of the torso, like us? It can't be as simple as tree-climbers moving back to the ground, could it?
It doesn't fit the standard vertebrate body plan, so the short answer is no, but the long answer is "maybe."
If you want an 8-limbed vertebrate, you have to start very early in evolutionary history. The 4-limbed body plan for animals arose around 500 million years ago, and ever since we've been stuck with it.
It's certainly possible to evolve 8 limbs, but there needs to be a selection pressure for it. Limbs are expensive to maintain, as they cost energy (i.e. food) to form. You'd need some kind of selection pressure to push for and maintain 8 limbs. You can see this in the fossil record, as many animals that are bipedal give up their forelimbs, Tyrannosaurus sp. and flightless birds being a prime example. So even if you start out with 8 limbs, there's no guarantee they'll keep all of them.
Note also we've never had a situation where more than 4 limbs were added through evolution, which is why you have to start early in your body plans.
That said, even if you can preserve the 8 limbs, what you're likely to end up with in a humanoid is something more like a centaur, with 6 legs and 2 arms, than what you're looking for, with 6 arms and 2 legs, so you'll need a way to push for bipedal motion.
Another side effect is that it won't be just your humans that wind up with 8 limbs, but all vertebrate animals. The Anime/Manga "A Centaur's Worries" is interesting in that it captures this fact properly: all vertebrates have 6 limbs. Humans might have two of those limbs become wings, or they might be a centaur, but the limb count still adds up to 6. In addition, even dogs in that universe have 6 limbs, so do cats, birds, etc., so it's obvious the 6-limb bodyplan came early in that world's evolutionary history.
Yes, through the mutation of Hox genes.
You do not need a long evolution to develop an 6-armed humanoid.
Hox genes specify regions of the body plan of an embryo along the head-tail axis of animals.
Mutations in the Hox genes can result in body parts and limbs in the wrong place along the body, missing limbs, or even duplicate limbs.
One Hox gene experiment on the fruit fly stated "In Ubx loss-of-function mutants, Ubx no longer represses wing genes, and the halteres develop as a second pair of wings, resulting in the famous four-winged flies. "
While it is true that all vertebrates share the same basic body plan of 4 limbs, this is in part because the Hox genes are so well-protected against mutation, and also in part because a drastic Hox (i.e. body-plan) mutation is unlikely to be beneficial and propagate.
If something allowed Hox genes to mutate frequently, and you got very lucky, a humanoid could get a Hox mutation for 6 arms in as little as a single generation.
However, this would probably be of limited utility (and possibly even painful), since the rest of the body is not built to deal with this body plan. I speculate that there would most likely be some kind of severe extra strain/torsion on the spine, to start with.
In order for this mutation to survive and continue, the 6-arm body plan would have to provide some kind of specific survival benefit, so it would be naturally selected for.
Over a long time (or a shorter time, with additional luck), there would be further mutations to accommodate this radical change, limiting some of the drawbacks.