Given a table Tbl tbl, you can access its content with tbl.getContent() which returns List<Object>. In the simple case, these are table row objects (Tr), but other stuff is also possible. For example, a table row can be wrapped in a content control.
To add a "column", you need to add a table cell to each Tr object. (You do that by adding a tc to the Tr object's content list). See TblFactory for an example.
You should also add a new gridCol entry, as shown in that code.