Question about getting into the DLC \ "Old Hunters \" (Bloodborne)

Friends, not too long ago I bought GOTY "Bloodborne" which includes "Old Hunters". I didn't really get into what this DLC was like - I just went through the game.
Now I already know that the entrance to the location is a DLC rather peculiar, but that's not the point.
Tell me, now I need to kill the First Hunter (as far as I understand, this is the end of the game?), If I do this, can I get to the DLC locations? Or will I have to start the game over?
Wouldn't it be better to spit on the Hunter for now and head to Hunter's Nightmare?
Thank you in advance.