it sort of depends what you mean by 'superimpose'...?
if you want to bring a flash-animated texture into anim8or and have that on your model in a 3-d scene, then you use terranim8or, like 3d joe said. there's probably a tutorial on terranim8or animated materials out there somewhere.
if you want to import a render of your anim8or head model into flash, and then use flash to animate details in 2-d, then it becomes purely a question about flash technique, really, as the render will act just like any other image when it's imported to flash. if that's what you're after, then we're probably not the best people to ask...
- colclough