Depends on whether I'm using it for flavor or to compensate for undershooting gravity and not catching it early enough
I usually add in the kettle in the last 5 minutes or so, maybe at knockout if I'm afraid it will scorch. My kettle has a false bottom, so if I use liquid sugars I don't like to just pour them in while the flame is on.
If add them to the fermenter, it's near the start of fermentation.
Adding sugars at the end of fermentation sounds like priming to me. Yeah, I've done that... Sometimes you want to add interesting sugars for priming when you want maximum flavor carryover. De Dolle Stille Nacht primes with honey, for instance. I've also primed with Lyle's, brown sugar, molasses, treacle, etc.