They aren't actors, so I don't think it's possible.
They are creatures and, should be able to be set to respawn. I've done some playing around with no luck! "Encounter Zones" don't seem to work the same as they did in FO3, after setting up a custom "Zone" with what I thought was a one day respawn, my effort fell on its face.
I like the ravens, I've even used them as an encounter in my upcoming "NVInteriors Project". I was hoping they would come back after several days so, I'm going to have to look into this further.
cev