Export not allowed with an imported video... why?

I am using normal mpeg videos, I seldom run across FLV video. Also, I'm using an up-to-date Animate, but yes I do remember I could do this back in the CS days.  Maybe I could translate the mpeg into a FLV and then export that. But aside from the probable quality loss due to multiple compressions, if I'm to add a passage I might as well just stitch the animation and the videos together in an editing app. Still, pity that simple feature was ditched.