If you have custom made models you have to rig them, animate them and import them. You can find tutorials for that here:
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In the future UGX will have a tutorial for the process on this but for the time being the link above will have to do. To import the weapons to world at war you need maya 8.5 or blender to rig/animate/export the weapon models and animations. For removing/adding weapons to the box move the script "dlc3_code.gsc" from root/raw/maps to "root/mods/*YOURMAP*/maps". Open it inside the pasted location and search for "include_weapon". You should see a bunch of instances of something along the lines of "include_weapon("zombie_colt");". That is just an example but if you wanted to disable that weapon from being in the box all you have to do is add a second parameter to that function that just says "false" like so: "include_weapon("zombie_colt", false);".