I don't think you can opena file directly into the Flash video...due to security restrictions...I believe you have to use the FileReference class in conjunction with some server side scripting for the user to upload a file to your webserver, and then have flash read that file and do with it whatever you need.
As for opening another swf with FLV in a popup window....you can do one of two things:
1.) just create a new html page and use the navigateToURL function to open it up when you need.
either way works just fine...I prefer the ExternalInterface myself, because it looks a little nicer.
Peter: "Look, my alphabet soup has a message in it! It says 'ooooooo'!"
Brian: "Peter, those are Cheerios."