Various parts of the sipXcom system are setup to run code written to a specific specification. Code could compiled Java, C++, or even shell scripts. Plugin specifications for sipXecs native code and embedded applications like FreeSWITCH, Openfire and OpenACD are all available to you.
Sometimes ingenious folks figure out how to change the system where no one had envisioned it could be. This is hacking in the creative and useful way.
What powerful about the plugins is the rich deployment capabilities. After you've developed your plugin, you can use the sipXcom build infrastructure to bundle your plugin as an RPM and deploy this on your network or bundle it into a custom CD.