I was wondering if someone can tell me how I can add an editable cell in the list of items in the drop down list?
It's not altogether clear what you mean. There are two basic ways to create lists of items for drop down lists. In the first, you simply enter them into the Data Validation dialog box. In the second, you create a list as a column of entries and reference that set of entries in the Data Validation dialog box. In the latter method, it's quite easy to edit any or all of the entries. (There's a hybrid of this latter that involves the relatively new Dynamic Array functions, so that the entire list becomes a function of a separate column on a table in the workbook.)
Somehow, though, I think you're looking for something else. So could I ask you to spell out a bit more fully what you're trying to create.