Firstly, download the model and place it in 'cstrike/models/player/name/name.mdl' and change 'name' to whatever you want, for instance Admin.
Open up your 'cstrike/addons/amxmodmx/configs/zombieplague.ini' file and go to the '[Player Models]' section, PS: This and the zombieplague.cfg are your main files for ZP cvars and adjustments, extra's go in zp_extraitems.ini and classes in zp_zombieclasses.ini.
Once you're at [Player Models] which is near the top there will be a list of things such as:
[Player Models] (randomly chosen if more than one)
HUMAN = umbrella_human_two
NEMESIS = nemesis
SURVIVOR = survivor
ADMIN ZOMBIE = zombie_guerilla
ADMIN HUMAN = robocop
If you'd like to globally edit the admin model to make it be the same for ALL classes change ADMIN ZOMBIE to the name of your model, e.g. ADMIN ZOMBIE = admin_zombie.
As you can see you can also edit Survivor, Nemesis, Human and Admin Human models.
To edit classes to have specific models go to your zp_zombieclasses.ini and find the line saying 'MODELS = zombie_source'.
Change the zombie_source to the specified file you want.
Note: Character models MUST be in the player folder and NOT in the zombie_plague folder in models unless specified.
Hope this helps, yours sincerely