> I will try this method when I upgrade to ThumbsPlus Pro Version 10.
Well, when v8 was released, the IPTC-editor was replaced by a different command, Image | Batch Edit Metadata. That's still how it is with version 10. You would use it with these settings:
[*]XMP field to modify >
Keywords[*]Operation to perform >
Append[*]New value > Get value from > Image info >
Keywords[*]Click 'Add step', Press OK.[/list]