There is a work-around, but it's not perfect. Open the Q menu, go to the Weapons tab, right-click the weapon you want, then click on "copy to clipboard". Next, open up the ZINV Editor, and in the place where you choose a weapon for your NPC, right-click on the blank space and choose "paste", or simply press Ctrl + V on your keyboard.

Now, the reason i say it's not a perfect work-around is that most modded weapons will be glitchy, and won't work properly when used by NPCs. It sucks to admit this, but you're better off just playing with the default NPC weapons, as they work better than most mod weapons.