*butter fingers appears in a cloud of purple smoke*
You mean, you want to put a hand on one of the stock weapons or, you have a weapon with a hand already and you want to change the texture.
If it's the first, you need to make your own hand model, in a 3d program then import the stock gun so the hand is holding it. (that will be hard)
If it's the second, navigate to gamecore\guns\GUNYOUWANTTOCHANGE.
open the guns dds texture file in gimp or photoshop or whatever (cant use paint), and change the hand texture. that way is easy.
Goodluck.
If you have any trouble, get back to me here!
*Dissapears in a cloud of pinkish yellow smoke*
"Guns for show, knives for a pro."