First and foremost- Can a Skald select Rage Powers from the Unchained Barbarian?
Second,
"When starting an inspired rage, the skald chooses which rage powers (if any) to add to the song, and all affected allies gain the benefit of these rage powers, using the skald's level as their effective barbarian level."
If I took the Superstitious and Witch Hunter Rage Powers, could I apply the Witch Hunter rage power to my song without applying superstitious? For obvious reasons.
The next paragraph says this:
If a rage power requires another rage power (such as disruptive, which requires superstition), the skald cannot grant that rage power to allies unless he can also grant that power's prerequisite.
So no, you have to grant Superstitious if you want to grant Witch Hunter.
I don't know about the first part, but my assumption is that it's a rage power, so as long as your GM allows the resource, then it's viable.