Hi Mike,
Thanks for your offer, we definitely appreciate your help.
The best way to start contributing to the project is to file an issue on the project issue tracker (
http://code.google.com/p/gdata-java-client/issues/entry) and attach a patch with your code. One of the project owners will review it, approve it and commit it into the repository on your behalf.
Once you start contributing on a regular basis, you might want to request write permission to the repository so that after the review process (we always do that) you can directly commit your code.
For what concerns your specific change, adding multidomain support to the Email Settings API is a common feature request and we are already working on it, so you may want to wait for our next release instead of investing your time in the same effort. Of course, please feel free to send us your code if you have already assembled a patch and want to propose it for inclusion.
Thanks
Claudio