They go in:
Docs and Settings/user/Application Data/Adobe/Dreamweaver CS5/lang_folder/Configuration/TagLibraries.
\\Library\Application Support/Adobe//same as above.
And yes, you can make an extension that adds things to tag libraries.
You will have to create .vtm files for it.
If you want an example, check out the HTML5 tag library that I wrote here:
If you install that, you will see the ..../TagLibraries/html5 folder in there. That's the new stuff.
Hope this helps.
Thanks Don - its much easier and quicker to add tags directly into the file rather than the Tag Library editor.