Can't produce tables with merged cells "from scratch" using the native Notes classes. The Notes C API or the Midas rich text LSX from www.geniisoft.com can do it.
Alternately, the chapter on rich text programmability in
this redbook shows how to create the table elsewhere -- in a form using Domino Designer or a rich text field using the Notes client -- and copy it into your application, replacing the cell contents with your desired data. See page 714.