metadata - How to add keywords to lightroom from a textfile (after import)?

I have a fairly large (20,000 photos) catalog. I'm trying to add keywords from the exif data that lightroom doesn't read, for instance any effects (like "pop color") applied to the photo.

Using exiftool I extracted all the data I want, and now I have a long text file like:

./2012-12-30/DSC03532.JPG   effect-Partial-Color-Blue

./2012-01-07/DSC00353.JPG   effect-Partial-Color-Yellow
...

And now I'm trying to figure out how to get that data into lightroom as keywords. I know I can write the keywords to the files directly and then have lightroom import them, but doing so wipes out all existing keywords I have on the files in lightroom.

Any insight?