Add-on are tackled also in Developer's Guide.
The addon are like components but they don't add a particular interface. To put it differently they need no IDL file for their construction. Then, if you want to call an addon only menus are available. To describe such component we start with the SDK example in
Let's begin with a little comparison between the architectures of a component and the architecture of an addon.
This figure shows us what are the interfaces we have to implement. Four interfaces are showed : com.sun.star.frame.XDispatchProvider, com.sun.star.frame.XDispatch, com.sun.star.lang.XInitialization and com.sun.star.lang.XServiceInfo, and it's a good idea to have a look at them.