Unfortunately weapons have to be "rigged" to a hand model so in fact weapon+hand(+arm) are kind of a single model.
If you want to introduce custom hand models you have to rigg those to each single weapon you want to use in a game.
At least you can change the textures of weapons and hands independent from each another.
Model Packs 5,6, 9+10 either have different textures included or the creators provided some later on.
Use search in the "Models and Media" board with either "Model Pack xy, "MP xy" or simply "hands" to check out what is available.
Hope that helped.
In case you find my grammar and spelling weird ---> native German speaker ^^
