The easiest way to dub over your video would be to create a new layer above what you have already imported. Then select all the frames of the new layer, right click, and select "convert to key frame". This will allow you to dub and edit frame by frame along with the imported video on your time line.
If you absolutely must have your movie in separate frame by frame on the time line, try this. (I am using Flash 9 so I hope it works in Flash 8 too. I don't know much about version 8.)
When importing your movie, just before the last step on the Encoding page, select the Video Tab and type "1" into Key Frame Interval. It places a key frame in every frame of the imported video.
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