Hi Joao,
If I'm understanding your description, you have made the list in column B of the table shown, then selected the cells containing the list and changed their format to "Pop-up Menu".
So far, I see no issue.
For simplicity, I'll assume you've placed only one copy of one of the pop-up menu cells on one other Table.
You selected one of the cells containing the menus and Copied, then Pasted it into a cell in another Table (located on another sheet).
At this point, you have two copies of that menu: the original on the first table (plus the ones in the other cells used to make the original), and one made by pasting a copy of the original into a second table.
Each iteration of that menu is independent of all the others—there is no connection between them at all.
Changing one by adding (or removing, or editing) an item has no effect whatever on the other copy.
Using Numbers 3 or later, you can edit one copy of the menu cells in a group, then select the whole group and, using the Cell Inspector, Merge the menus so that the new items are added to all of the group.
Note that a merge will not delete items from the other menus, nor will it change items that have been changed in the edited menu. All items that are missing from any of the menus will be added to each menu that is missing that item.
You may want to do some experimenting (as I did) to determine the effects of various changes to individual menus.

All cells containing data started with the list in B2:B5. After pasting three into D3:D5 (D5 selected), ITEM A an ITEM B were added to one menu, and in a separate edit, Item 2 in the menu n B4 was edited to Item K.
A merge after each change led to all seven cells containing the menu shown.
Regrds,
Barry