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?