Adding Finger charts to existing Muscore pieces

I teach tin whistle using Muscore. I have the tin whistle plugin and use it on NEW pieces and it works great. However, I want to take an existing piece in Muscore and add the fingerings using that plugin. When I add instrumentation to the existing piece it erases all the notes. I have to go back in to add the instrumentation and start fresh in the setup before I place any notes in order for it to work. So how do I do this without having to start all over again?

Thanks,
Larry Halverson