I found several ways to do this. First thing to try is copying the HTML text to your clipboard (Clipboard.SetText("hello"); -You'll need to use the System.Windows.Forms or System.Windows namespaces for that) and paste it as HTML with PowerPoint.TextRange.PasteSpecial()
] where you can set the format of the text.
I don't know if this workaround will do the job because in difference to word, powerpoint documents are not based on the HTML language.
Marco Alessandro Bertschi