The only possible way to do what you want is by creating the text element in another program like Photoshop,save it as a PNG file and then import that in InteractBuilder as a Graphical Element.
Somehow, sometimes while entering text, I will start a new line and it has become italicized. I am not sure how it is doing this, I backspace up to the last line, press return again, and it returns to regular. So there is some method it should work, but I haven't figured it out.
But you can make the text underlined by adding a Shape element ( a line) and add in below the text element.