My guess is that you can probably fudge it using CSS. But it won't be something RoboHelp will assist with in any way.
Check the link below for one hit I found. Use Google searching for vertical text in HTML to find others.
Helpful and Handy Links
You can do that for IE, FF and Chrome/Safari in your style sheet. For Mozilla, use -moz-transform and for Chrome/Safari -webkit-transform. For IE, you need to apply a filter. See http://snook.ca/archives/html_and_css/css-text-rotation for an example.
RoboHelp won't do much to help you and the actual transformation is different in every browser
Using images might be easier.
Hello again Willam,
I think you're right; images would be easier!