Not easily. There are three routes and none are good. One is to use classical frames. This isn't standards compliant and future browsers may drop support for the page since frames are deprecated.
The other path is to not actually change the page and use AJAX to bring in new "pages" - the largest problem with this is the browser back button and bookmarks will no longer work for the site.
The last route, and the only one I'd consider, is to use a pop-up window.