Can you describe your problem with a little more detail...?
in Numbers for iOS, you can:
Select a cell, use Format -> Pop-up Menu
If the cell currently has a Pop-up Menu defined, you see its options and can add new ones.
If you select multiple cells that all have the same pop-up menu assigned, any edits are applied to all selected cells (e.g. a new menu option is added for all cells). Existing cell values should be retained.
If you select multiple cells that have different pop-up menus assigned, Numbers collates all possible options and creates a new pop-up menu containing all the combined options. Again, existing cell values should be retained.
If the selection contains cells that had no pop-up menu assigned, the menu is assigned to that cell.
The only case where existing cell values would be removed is if that cell's option was removed from the pop-up menu.