How to add new entry in a existing combo box?

I want to add new entries in a combobox. In the property section of comboBox i found "value list" mentioned as "$G.Cust.Flag". 

to add new entries i tried the below solution: (by adding new value in the value list, but its not working)

I am not able to find the mentioned variable ("$G.Cust.Flag"). as per my understanding i need to add new value inside this mentioned variable. How can i achieve this?