As far as I know, the construction of the Rebel or that of any other SLR with a flash built onto the pentaprism will not allow you to do what you want.
When the built-in flash module pops up, it prevents the installation of an external flash unit.
See-http://www.dpreview.com/reviews/canoneos400d/page6.asp
The only way that I'm aware of providing both direct and bounce light from a flash mounted in the hot shoe would be to use one of the Metz units or another of similar dual flash tube design or to use an attachment like-
http://store.garyfonginc.com/liiido.htmlI use the Fong unit all the time for my event work and it provides a beautiful light on the subject, and allows one to eliminate or at least minimize the shadows behind the subject. This does come at the price of substancial reduction in light output, however.