For backup, your CS5 encoding presets reside here:
I am not running CS5.5, but I assume a corresponding folder
will appear in your \Common\AME\ directory on installation.
What JBP says is correct; you can just copy and paste them from the CS5 folder to the similar CS5.5 folder. You can also use the Import Preset option in the Export Settings dialog to do this on a case by case basis; just navigate to the same CS5 path, and then you'll have the option to save it as a CS5.5 preset.