Hi stefazo,
"I would like to automatically add some cells to each sheet"
Can't be done 'automatically'. You need to add new rows or columns to the Tables manually, using the row and column controls visible at the right and bottom ends of the column and row reference tabs displayed when the Table is active (one or more cells selected), or by selecting a cell, then pressing option and tap one of the four arrow keys to insert a row above or below the selected cell or insert a column to left or right of the selected cell. The new row or column will be added in the same direction relative to the cell as pointed to by the arrow key you tapped.
Regarding your original question: "how can I insert a sum of cells from different sheets?"
Start by giving each of the Tables containing the cells to be summed a distinct name. This will release you from having to include the Sheet name of the Sheet containing the Table as part of the address for each cell or the range of cells to be summed.
You haven't given any indication of which cells contain the values to be summed, so I'll assume there are three cells, one on each of three tables, Brand A, Brand B and Brand C, and that each cell is at the same address (row 6 of column D) on its Table.
The sum is to be returned to a cell C7 on a fourth table, named Main.
Select cell C7 on Main by clicking on the cell once.
Type = to open the formula Editor, then enter SUM(Brand A::D6,Brand B::D6,Brand C::D6)
Click the green checkmark to confirm the formula and close the editor.
The sum should appear in Main::C7, the cell containing the formula.
Regards,
Barry