StarOffice8 can open .CWK documents. This is part of the W4W proprietary filter. So.. technically it is possible already.
You can get StarOffice as part of GooglePack http://pack.google.com/intl/en/pack_installer.html?hl=en&gl=us
but... it's only part of the Windows Pack... it's not included in the Google apps for Mac.
So... a possible solution is to install StarOffice from Google Pack on a Windows PC, import all the CWK documents you have, and save as ODF. It would be a one time conversion, and then you'd be able to work on your old documents.