To Add a Cell to a Custom Table:

Help > Authoring > Working with Tables > Table Editing > Inserting and Deleting Cells > To Add a Cell to a Custom Table:

1. To add a cell, place the cursor in a cell and choose Table > Insert > Cell Left or Table > Insert > Cell Right. The cell containing the cursor moves in the direction specified and a new cell is inserted.

If more than one cell is selected, the selected cells move in the direction specified and the new cell is inserted.