There is no option for this within RoboHelp although it might
be possible to tweak some of the output files after generating
WebHelp. Perhaps someone else can provide specific advice for that
Another idea would be to make your own TOC. In other words,
don't use the TOC built-in to RH. Just make a topic page and
manually create a TOC. Then add a link (as a snippet) to the top of
each page linking to your TOC.