Quit Numbers if it's running, then control-click on the Numbers application icon and choose Show Package Contents from the pop-up menu. Type date into the Spotlight field in the Finder window, and click on the DateTimeFormats folder. Alternatively, drill down into Contents » Frameworks » SFUtility.framework » Versions » A » Resources » DateTimeFormats.
Control-click on en_US.plist (or your language's plist file) and click Duplicate to make a backup. (Just rename the backup if you damage something.) Open en_US.plist (or your language's plist file) with a property list editor. I use Property List Editor from the Apple Developer Tools, but I think Pref Setter is also easy to use. Expand AcceptedDateFormats and check if your desired format is there. If not, add a string key with the desired format. Open DisplayedDateFormats and add a string key in the format you want the date displayed.
Save the plist file and restart Numbers -- your desired date format should appear in the list of formatting choices. (This also works for time formats.)
[robg adds: If someone manages to find the counterpart to this hint for custom number formats, please submit it! I searched quite a bit, and didn't see anything that looked like it would work.]

