[ZaX]

I did this for myself if you want to use it

#include <amxmodx>
#include <zp50_class_zombie>
#include <zp50_colorchat>
#include <crxranks>

enum _:ClassesInfo {
    szClassName[32],
    iLevel,
    szClassID
}

// Add new class like shown below
// { "Class name from the original source", Level required}
new g_aZombieClasses[][ClassesInfo] = {
    { "Vampire Zombie", 25},
    { "Assassin Zombie", 32}
}

public plugin_init() {
    register_plugin("[ZP] Zombie Classes with levels", "0.1", "X3");
}

public plugin_cfg() {
    for(new i = 0; i < sizeof(g_aZombieClasses); i++) {
        g_aZombieClasses[i][szClassID] = zp_class_zombie_get_id(g_aZombieClasses[i][szClassName]);
    }
}

public zp_fw_class_zombie_select_pre(id, classid) {

    if(!is_user_connected(id))
        return PLUGIN_HANDLED;

    new sText[32];
    for(new i = 0; i < sizeof(g_aZombieClasses); i++) {

        if(classid == g_aZombieClasses[i][szClassID]) {
            formatex(sText, charsmax(sText), "Level : [\r%i]", g_aZombieClasses[i][iLevel]);
            zp_class_zombie_menu_text_add(sText);
            if(crxranks_get_user_level(id) < g_aZombieClasses[i][iLevel]) {
                return ZP_CLASS_NOT_AVAILABLE;
            }
        }
    }
    return ZP_CLASS_AVAILABLE;
}

You can get zombie class name either from the plugin source, its usually defined at the top.
Or from the file zp_zombieclasses.ini
Here is an example of the zombie class name, obviously dont use the classic zombie