Sorry to be the bearer of bad tidings. No, RTF does not support lines, checkboxes, etc. I had to redesign a report because of this and so I know how infuriating this can be.
I read somewhere that you can simulate a line by adding a label (or text) box under the text you want to underline and fill the box with the underscore character. (Set the box to the width of the text to underline. I think there's a function that will fill the box with the character you specify.)