Sorry for bringing up this old thread, but such a feature would be really awesome.
I think the only challenge here would be, that such a feature should compare the instruments of the imported song with the instruments of the existing song(s). It would reuse already existing instruments, add the new ones, and remap the instrument IDs in the pattern data.
Maybe I can write an own python tool, which is just loading the two XML files of the AKS musics, compare the instrument parts and write a new one, but I am not sure, if I would get it all and manage it (xml is some time ahgo, since I am only working with Json now).