Physically possible, yes. Does it make sense for it to evolve? No.
More info on physical possibility:
Snakes are capable of moving bones (or any hard object) in the way you're talking about. Your worms can take biological cues from them. Plenty of creatures build nests, pouches, and storage areas of various kinds in various ways. Your worms can create "organs" in a similar manner, and a rib cage is well suited to build them on. The same mechanism could create a skin-analogue. The only possible barrier I see here is looking like a skeleton. Anything in the real world capable of animating bones is going to be large enough to be visible. I don't see how you could have this colony animating a skeleton and not being visible enough to never be mistaken for a living skeleton. Maybe it could be mistaken for a zombie, though?
More info on evolutionary problems:
I can't think of any collection of environmental pressures that could cause this sort of creature to be selected naturally. First, let's look at what might select a creature that's attracted to the bones of another creature. Scavengers spend a lot of time with bones, but only until they're picked clean. I could also imagine a creature like a termite making its home inside bones or maybe even using the bones as a food source if calcium was scarce in its environment. For either creature, though, I can't think of any step toward using those bones for anything other than a protective shell, a weapon, or a nest that would be remotely useful for survival. Even if we get past that hurtle and the creature or colony of creatures for some reason starts carrying bones with it and not fashioning them into one of the previously mentioned items, I can't think of anything at all that would help them evolve toward animating the bones in the fashion of the bones' previous owner. It just doesn't make sense to spend the energy required to do that, except MAYBE as a disguise or scare tactic, but unless these creatures are very intelligent and very motivated, I don't see them getting to that point. (Intelligence would explain away the need for evolving an instinctual desire to animate the skeleton that way, but they would still need a good motivation to expend the energy required to organize and carry out the plan) I don't think you're designing a creature with that kind of intelligence, though.
That leaves open a few options, in my mind:
1) Ignore the evolution issue. There's a lot we don't know about evolution, anyway, so I think it's okay to just leave its evolution a mystery.
2) Evolution doesn't exist. Maybe your world and its creatures were intelligently designed by a god. Maybe ours was. There are arguments for that. Let's not start a religious debate, please!
3) This creature in particular did not evolve naturally. It could have been genetically modified or created, or magically modified or created. Maybe some cult selectively bred it over millennia for who knows what reason. Go nuts! (Much like the aforementioned cult)
That's all I've got. I hope these thoughts help :)