Unfortunately, you cannot change that behavior. Sometimes, those buttons can be triggered with a Key Command (i.e. command key plus the first letter of the command). It works, for example, with the Replace command in many Dialog windows. But it doesn't work with this window.
Your only option would be a Keyboard Utility like QuicKeys or Keyboard Maestro. You can program a mouse click action on the Keep button and assign it to a key combination of your choice, because this Dialog window always pops up at the same screen location.
Hope that helps
Edgar Rothermich - LogicProGEM.com
(Author of the "Graphically Enhanced Manuals")
http://DingDingMusic.com/Manuals/
'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'