You can't, as far as I'm aware, directly embed a swf as a background. Either turn the original swf into a flv or copy-paste the frames required between your projects from your .fla file.
Flash CS4 and FLex 3.5 + have the Embed syntax that links an external SWF into your main swf at compile time. This swf can then be accessed by the code in a similar way (though not quite the same) as a library symbol.