I mean why are you removing the folder with the negatives and then trying to re-add it later? Understanding your goal may help me understand what the best solution would be for you.
If you remove files from Lightroom, and then re-add them later, they will lose the settings that you had associated with them, unless you have saved the metadata to file or into an xmp side… even if you retain the settings in that situation, you would lose the Negative Lab Pro analysis information that links the plugin settings to the files… that info is stored in the Lightroom catalog itself so when you remove the files, you lose that.
If you want to move the NLP-edited files from one computer to another, for instance, you need to export as a catalog, like shown here:
Does that help?
-Nate