Hi Juan Antonio!
This technique is for using native code from your Rosjava program. It will work in a similar way if you are in Android or not.
First you need to compile your native code using the Java Native Interface specification. To learn how to do this, you can see for example this. You should compile and link your code to generate a shared library targeting the system you are working on.
After that, you need to write a java class extending the NativeRosjavaNode. This class must have defined native methods that match the exported functions in the native library. This way Java will redirect the calls to those methods as calls to the library functions. You load the library passing the a string containing the name of the library to the NativeNodeMain constructor.
I can implement a simple example if you want.