The easiest way I've found to do it is to:
find a launcher in the same place on the 'start menu' as the one that you want to add;
go to /usr/share/applications;
make a copy of the menu item in the same place (need to be root to do this), i.e.
cp redhat-web.desktop thunderbird.desktop
was one that i did to make a shortcut to thunderbird in the internet folder
edit the copy, it's quite obvious what things need to be changed to what for the most part
there's possibly a better way, but this is the way do it on my machine