Data Validation from multiple source

Hello
I have a name defined (Branches) and use data validation as
Allow
List
Source =Branches
It is working perfect.

I wanted to add another text called "South" which is not defied in the list "Branches".

So I need to allow data entry from the dropdown list (=Branches ) and "South" which is not in the list.
I do not want to see "South" in the dropdown list but it should accept "South" if I enter it manually
Could you please help me.
Thanks