I’ve made my way to Corvega Assembly Plant and have encountered a “Raider Scum” enemy in Power Armor with a Fatman. I’ve killed him and could loot his power armor parts, as well as everything he had equipped with the exception of his power armor frame which he seems to be wearing still.
I am aware that NPCs can steal power armor frames with an intact fusion core, but is there a way to get frames from (preset) NPCs?
No there isn’t, it’s not possible to loot the power armor frame from an NPC.
What you can do is pick pocket the fusion core from the NPC, which will force them to get out of the power armor. It is also possible to damage the fusion core while the NPC is wearing the armor, which will force them to vacate the suit as well.