Of course JAS provides plugin support for easy extension of the functionality. The plugin API provides a simple way of implementing new plugins and combined with the data and templating engine new functionality is implemented fast. Also the JAS backend integrates with the online plugin repository which allows you to directly donwload and install plugins created by others.

If you want to extend JAS' functionality it's generally best to do this by use of plugins. A good start for this is having a look at the Skeleton plugin.