Sorry to answer so lately, hopefuly this can however help someone else.
You can create hierarchical structures of meshes, cameras and lights through the scene graph system. See iSceneNode
Basically, you simply need to call something like:
childMesh->QuerySceneNode ()->SetParent (parentMesh->QuerySceneNode ());
Then the relative position of the nodes will be automatically kept constant. The sectors of the child node will also be updated when the parent is added/removed from a sector.
It seems that the XML loader cannot handle that, although this feature should not be hard to add.