I do not know Photoshop very well, but I know that you can import your video into After Effects, then export out an image sequence from there - TGA or other "Lossless" codecs. Then you would have individual frames to work on. Once done, you could use Media Encoder to get a good quality export from the individual images back to video.
Either export as an image sequence (long and hard to make smooth edits in Photoshop that way) or render a lossless or nearly lossless QT and open that in Photoshop and paint on the frames. That way you can at least view the movie you're working on. Editing individual frames without the ability to onion skin is nearly impossible.
There are several tutorials about creating animations in Photoshop.