I’m afraid it’s not, no. For the time being you may need to create another instrument in A, like a horn, but it will be difficult to fool NotePerformer into choosing a different sound. I believe a future version of NotePerformer will allow you to change the sound used for a given channel directly in its mixer interface, but that’s not possible in the version available now. We also intend to make it possible to define your own instrument types in future.
I’ll be honest, I’m not sure how I’m meant to use the example Dorico file and attached xml file. If I enter notes on Bass Clarinet in Bb and copy past into the Bass Clarinet in A they both produce the same note. I sense either I’m misunderstanding how to use the files you attached or Noteperformer is disregarding the xml changes?