you'll need to do that in the authoring environment.
or, you could leave your current fla with the old stage size, publish your swf and then load that swf into another swf which has the larger stage size. center the loaded swf and you'll get the effect you want.
Thank you for your suggestion. I tried doing as you said but when I try to import the swf into the new file it states that it is protected by it's author and can not be imported.
you should load the swf, not import it.
if you're using as2, use loadMovie(). if you're using as3, use the loader class.