goto root/raw/maps located here:

C:\Program Files (x86)\Steam\SteamApps\common\Call of Duty World at War\raw\maps
and look for "_loadout.gsc" 


copy and past it to here:
C:\Program Files (x86)\Steam\SteamApps\common\Call of Duty World at War\mods\mapname


open the _loadout.gsc with either notepad or notepad++ and search for the following:
zombiemode

you will find these 2 lines:

set_player_viewmodel( "viewmodel_usa_marine_arms");
set_player_interactive_hands( "viewmodel_usa_marine_player");

change these 2 parts:

viewmodel_usa_marine_arms
and
viewmodel_usa_marine_player

to:
mason_viewhands_bo2 on both lines

save, build mod enjoy your new arms :)