Menu list for cell in numbers

There are a few ways

1) Use the + button in the cell format sidebar to add a new item then edit the item to be what you want and drag the item to its proper place in the list.

2) In your table, put your pop up menu in one cell and your new item in an adjacent cell. Select both cells and format as pop up menu. it will add the new item to the bottom of the list. You can then use the format sidebar to arrange the order.

3) I usually create a pop up menu from a column of data in a table. I enter all the data, sort the column into alphabetical order (if needed), select all the cells, then format as pop-up menu. Then I copy one of those cells to use as my pop up menu to paste elsewhere. If you have such a column, you can add a new item to the column, sort the column, change the format of the cells to automatic, then change the format to pop up menu and it will create a new pop up menu with everything sorted. This is how I usually do it, especially if it is a long list and I want to add multiple new items.