Yes it can be done, for example: When the web site loads up an actor walks onto the website and introduces themselves. We have all seen it lots of time. I’ve done a lot of them myself. Anyways, for this example you would export out the keyed actor. Make sure it is set to Quicktime movie and that the codac is Animations (Set to Millions colors +). The flash designer will take that into flash and the transparent part of the clip shows through so you can see the flash designers background. Hope this helps.
Lone Peak Productions
Check out my DVD Money Making Graphics & Effects for Final Cut Studio 2