If you load in _level0 you will delete everything currently resident on all levels and replace it with the loaded swf i.e. if you want to transfer a variable you have to either write the vale externally or load into a different level.
It is possible to write to an external text file using a server side script such as php, asp, cfp, perl. The format is that you call the script using the getUrl() method and append you variables using get or post. you’ll also need write permissions (chkmod) on the text file you’re writing to.
It’s also possible (although I’ve never tried it) to append variables to url when calling a swf with asp and pass them into the swf that way.
Far easier is to load in to a different level - _level1, 2, 3, etc. or to load into an instance of a MC. The you can pass the variables to the _root or _level0 timeline as appropriate.