If you’ve got a photo that has keywords associated with it and you’d like to copy those keywords to other photos, try this. In Library grid view, highlight the photo that has the keywords attached to it. Then go to the Keywords panel. Highlight all of the keywords that you want to use there. Now press Ctrl + C (Mac: Cmd + C) to copy them. Go to the grid again and highlight all of the photos that you’d like to copy the keywords to. Finally, press Ctrl + V (Mac: Cmd + V) to paste the keywords in to those photos. Keep in mind that you can add keywords upon Import too, but if you don’t, at least you’ve got a way to do it after.