@RosenKreuz You can't add a border inside a single cell. The icon you circled represents a group of four cells and lets you draw a border an all inside edges of these cells. You need to select at least two cells for this icon to be enabled.
@RosenKreuz Don't understand the question. You an insert a column, you can merge cells in the header of two columns to make it look like as if you have a split column, but you can really split column B and call it column B&C.
