Dropdown without using namedset

Hi all,
I am using sytech for reporting purpose,in which the customer wants a dropdown menu for entering string parameter. The problem which i am facing is that, they need an add button along with dropdown menu,so that they can add n number of string values in future as dropdown. I think combobox would be one of the option in vba. Pl share if any one having idea for doing this with or without combobox. A big thanks in advance.
Regards ,
Mithun