You would presumably ask permission at Epic Games. However, they would say no, so you may just have to make it without their permission like everyone else who makes Jazz Jackrabbit games. That's the way fangaming works, really.
And don't just give it a number. Numbers are boring. Nobody names their level packs "Jazz Jackrabbit 7". Name it after whatever makes the game special, typically the plot. Like "Jazz Jackrabbit and the Sandwich Rebellion", or "Jazz Jackrabbit 2: Carrot Edition".