Sorry to hear that. It depends on where the ratings are stored. Did you apply the ratings in RAW Power, or in a different app?
If you applied them in RAW Power, then you will need to make a sidecar file that Nitro can read. You can do that selecting images in RAW Power and using File > Export Nitro XMP Sidecar.
That export feature is best used for small groups of files (not hundreds or thousands) – I did not implement a progress dialog, etc. I suggest testing it with a few files first. I hope to add progress and such in the future to RAW Power for exporting sidecars.