I would say: why bother?
I have been playing WFRP for many years and I can count the number of times a blackpowder weapon showed up on one hand. There was a blunderbuss in one of the published adventures and there was one session where our GM had a story about a crazed gunman with a prototype sniper rifle.
Gunpowder is not something you run into often in WFRP unless the GM purposefully writes it in there or one of the characters has one. Only Imperial humans and Dwarfs use them at all and even among them, it is a rare sight usually employed by specialized military units and nobles. Usually, people who wants a ranged weapon makes do with a bow, crossbow or throwing weapon.
You do not have to rewrite the Warhammer setting, you just have to be a bit selective about which parts you concentrate on. Which is what a GM does anyway. If you run the premade adventures from FFG you can just rewrite any guns that might show up to bows or crossbows.
Edited by Ralzar, 27 October 2013 - 03:19 PM.