After inserting a table in a document, it's always possible to edit it using the Edit tool. For example, you can select a row and use Insert After to insert after it another row or you can select a cell and use Insert Before to insert before it another cell. However the Edit tool does not make it easy working on table columns.
Fortunately, the DocBook, DITA Topic and XHTML menus all have Column, Row and Cell submenus which allow to perform all sorts of operations on table columns, rows and cells. For example, ColumnInsert After adds an empty column after the one containing the caret.
Note that precisely adjusting the presentation of a table may require you to use the Edit tool and the Attributes tool. For example, making the third column of a DocBook table twice as wide as the other columns requires you to insert a colspec element before the thead or tbody of the tgroup and then specify attributes colnum="3" and colwidth="2*" for the new colspec. However, when a precise column width is not required, suffice to drag the column separator as you would do it in any word-processor.
In the above screencast, the user pressed Ctrl-A (that is, EditRepeat) twice, one time to repeat Row Insert After and another time to repeat Cell Increment Row Span. If you don't remember which is the last repeatable command you have invoked, then you may want to use Edit Command History (Ctrl+Shift-A).