You may create a IRDHtmlTemplate and specify the "class=Heading1" and wordstyleid="Heading1" in the content.
IRDHtmlTemplate lText = reportDiagramDetails.createRDHtmlTemplate();
lText.setText("<html><body><p><span class=\"Heading1\" wordstyleid=\"Heading1\">This is the title may be</span></body></html>");
reportDiagramDetails.addTemplate(lText);
