You can create a dummy library as the (main) entry point, this dummy will load all your libraries (as SFML does), then when you finish loading all, call another entry point.
But if you want you can add a library for be loaded putting another "loadLibrary" down
this line.
Ej:
// SFML libraries
loadLibrary("sfml-system", lJNIEnv, ObjectActivityInfo);
loadLibrary("sfml-window", lJNIEnv, ObjectActivityInfo);
loadLibrary("sfml-graphics", lJNIEnv, ObjectActivityInfo);
loadLibrary("sfml-audio", lJNIEnv, ObjectActivityInfo);
loadLibrary("sfml-network", lJNIEnv, ObjectActivityInfo);
// Custom libraries
loadLibrary( "myCustomLibrary" lJNIEnv, ObjectActivityInfo);
You'll only need to rebuild sfml-activity if you go with the second method.