code: public void addCustomTrackingParameter(String key, Object value){
          if (key == null){
              throw new NullPointerException("Key cannot be null.");
          }
          if (value == null){
              throw new NullPointerException("Cannot add a null custom tracking parameter.");
          }     
          else{
              List l = customTrackingParameters.get(key);
              if (l == null){
                  l = new ArrayList();
                  customTrackingParameters.put(key, l);                
              }
              l.add(value);
          }
      }
docs: Add a custom tracking parameter to the specified key.  This allows users
  to have multiple parameters with the same name and different values,
  commonly used during situations where list parameters are needed
  @param key the parameter's key.  Cannot be null
  @param value the parameter's value.  Cannot be null