How do I add a new table to my extension?
What I did for the sepa extension is a bit of a hack, but worked ok:

write the xml schema files, as you have them on the (development version) core and run xml/phpGencode.php to generate the dao, that you then copy into the extension folders and voila, you got the tables ready to use (for a few more lines, you got the api files, worthwhile IMO)

The process is now detailed in our developer book