So, how does someone become a Valkyrie? Doing a little research about Norse mythology the evidence needed to answer this question is somewhat sketchy and some of it comes from Roman stories about Amazons and some of it is from the post-Viking age, both of which are questionable.
There is some talk about female warriors; shield maidens, who became Valkyries. According to Gard, the Maiden idea; as in being a virgin, is nonsense. I think that conversation is in one of the short stories.
With that in mind, I could see Murphy becoming a Valkyrie, if she didn’t get that status immediately. I could even see Jim doing a Conan the Barbarian movie reference that has Valkyrie Murphy asking / telling Harry, “Do you want to live forever?”