The only way to add e.g. "dog" to a group of files with DIFFERENT groups of keywords ( file1 "whippet, slim" ; file2 "chihuahua", "small" ) is to add it in keywords panel AND select it and permanently add it in the keyword list and structure.

It would be very useful to have the ability to add a SINGLE keyword on a group of selected files in the METADATA panel, not the keywords panel, like a"plus" sign, to add only this keyword regardless of the group of keywords already present and that are different in each file of the group.

Another way for nerds to get the same result is to use the command line utility exiftool, but it is not very practical in daily work.

(trying to add a JPG/PNG file but uservoice doesn't allow to do this , don't know why)