You can enable "treat repeating fields as a single string" so that it treats the entire keywords field as a single string rather than a list of comma-separated items, but In the event where you want to replace something like "Apple" and "Zebra" and you have one image with keywords "Apple, Zebra" and a second image with "Apple, Tree, Zebra", putting "Apple, Zebra" in the Fined field is still not going to replace anything in that second image, because there is no "Apple, Zebra" string in it.