Were I running 5e (or any other edition) NPC adventurers would use the same build rules as PCs. I'd houserule this in without a second's thought.
I'd absolutely do this, too. If for no other reason than my own sanity and the players' sanity. It's so much easier to use rules that already exist. E.g., random encounter gave the PCs a dog that the random encounter NPC gave to them. When the PCs decided to keep the dog it immediately was given PC stats. In this case, it was a lvl 1 Barbarian (Totem Barbarian at lvl 3. Wolf, obviously). 1/2 XP like a traditional henchman. Nothing new or strange for me or the PCs to learn. I already know how Barbarians work.