Do 60 fps for HD2 using 720p. Make sure enough ambient light. Do 60p for HD3 Black 1080p or 120 fps for its 720p.
Use Cineform Studio ( its free ) , transcode to Cineform AVI or MOV, ur choice depending on what editor u are using.
For the conversion MP4>>>AVI or MOV, use the setting and choose FRAME RATE 10, which is 6 times slower per second if from 60p source because 10 frames per second is what you get. Its decently smooth for a free program, even playing with 120fps of the HD2 480p, its not bad slowed down to 10 FPS, or 10 time slower.
From that Cineform MOV or AVI, you can use CineForm studio to convert to MP4 again as finished product. While at it you can play with color too ( limited ) or you can use any NLE capable of working with Cineform MOV or AVI. Cineform Studio is free and buggy sometimes.
This may help on the Cineform MOV or AVI compatibility
http://macnews.overblog.com/search/cineform%20MOV/
Good luck trying.