There's only scant mention of extraterrestrials in the Yarbro books, and I don't recall much reference to them in the later books, either. As I've said before, the focus of the teachings is not about extraterrestrials. If they're mentioned in the channeling, it's usually on the peripheral.
When you say extraterrestrial souls, do you mean souls with alien origins who are currently incarnating on Earth, or aliens that are visiting us?
Best,
Dave