Is it possible to un-equip power armor frames from NPCs?

Solution 1:

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.

Solution 2:

Actually, pickpocketing the core from the person will make them remove the frame, friend or foe.

However on a dead NPC you cannot currently loot the frame (Assuming this will be patched eventually)

Solution 3:

You have to pickpocket their fusion core while they're still alive. Once you do this they will immediately get out of the frame. Kill them, then take the armor. Although every time you get in it says steal. Hopefully someone else can explain how to get rid of the "steal" issue.

Solution 4:

I just fought slag, during the battle I was able to get him to turn around and shot at his fusion core. He hopped out of the suit while it was blowing up and after the fight the power armor suit was still standing there ready for me to pilfer. My advice is: if you can get them to turn around then shoot for the core: you'll get a nice suit and do some great damage, though the suit does say "Steal" every time I get in it which is a bit of a peeve but not that big of a deal.