You can do that pretty easily in Excel. It does "pop up" menus differently. The "items" can be listed in a table and, when you add an item to the list, all the "pop up" menus based on that list will automatically include the new item. All you need is a table of the items you want in the "pop up" and to set up Excel's data validation wherever you want the "pop up".
You can do something similar, but not quite, in Numbers but it is a few steps. In Numbers, you can create a pop up menu from a list in a table (which is the easiest way to do it anyway). Unfortunately, though, new items added to the list will not be included in existing pop up menus. Also, you cannot paste it over pop ups you are already using or it will change the selected item. You can only copy/paste it to cells with unused pop ups. So you will end up with different cells having different pop ups. I think with AppleScript it may be possible to overcome this limitation.
Create the initial pop up menu
- List the items in a column of a table
- Select them all
- Format as pop up menu. All will become a pop up menu of all the items.
- Copy/paste the pop up to cells that need it
Add new items to the pop up menu (similar procedure can be used to delete or rename items)
- Add new items to the column
- Sort the column (if needed/desired)
- Select all the items
- Format as text, number, date or whatever, depending on the type of data in the list
- Format as pop up menu. The new pop up menu should be in sort order.
- Copy/paste the new pop up to the cells that need it, overwriting not-yet-used old pop ups. Do not do this to the ones already being used or their selected item will get changed.