Volition did some weird shit with the customization. It's obvious the game was rushed badly.
There's no way to add nor replace the original animations, unfortunately. All flying NPCs use the same flying animations - Jezebel doesn't have them because she uses a special "jezebel" flag which allows her to keep small wings and of course Volition was too lazy to animate them. It is possible to replace them with the regular Winged Militia wings by removing "jezebel" flag but this doesn't look too well. You can also remove "wings" flag to make her unable to fly and add "has_super_powers" flag if it's so annoying to you. There's no way to replace regular wings animations with gliding.
You can replace NPC models with SR IV ones as far as I know but you can't add new ones, just replace. And there are less than 5 pedestrian models in GooH so...
I never tried replacing audio so I won't help you in this case.