If you are really interested have a look here:
http://ftp.dlink.ru/pub/NAS/DNS-327L/Add-on/Basically, you need the files that are name SDK.
Building an addon it's relatively simple if you know a little about linux. What the addons for this nases does is symlink the files for the program to the core os of the nas (linux), and using a set of files that tell the nas to autostart the program, stop it or delete it, etc.
About packaging, inside the file is a 32bit binary that packages the addon to a compatible format for the nas. And as far as I know, you can´t extract it in linux or windows.
If you say there are some remnants of the addons code in the 320 firmware, maybe you can look on how to enable it.
Hope this helps you.