After thinking further on the issue, it can be done, but it takes more than one step.
1) Apply IPTC Stationery Pad with: "{keywords}" in the Source field (it can be any field, but I figured you might not be using this one) to your selected images.
(your current keywords will now be in the Source field.)
2) Apply IPTC Stationery Pad with: "Australia, {source}" in the Keywords field and turn off the append (+) checkbox and the Source apply checkbox off to your selected images.
(your keywords are now as you wanted.)
3) Apply IPTC Stationery Pad with: Keywords unchecked, Source checked and its contents cleared to your selected images.
(the Source field is clear again.)
Not elegant by any means, but much better than entering the keyword 4,577 times by hand.
HTH,
-Kirk