Cool. Yeah the problem is if you have a bunch of objects in your current fbx scene then you merge another fbx scene, there is no way to distinguish the merged objects from the objects that were already in the file. You can get the filename from a user input but you would still have the problem of not knowing which objects to attach it to.
What you could do is create a temporary namespace for all of the objects in the scene, then merge the file, then look for all the objects without a namespace, then attach the file namespace to those objects, then remove the temporary namespace. Seems like that would work but would be slow.