I think what you are refering to is the threaded plug that the bolt attaches to. I think it is epoxied in. Have seen some adapters that replace the bar end caps and allow the mirrors to attach.
ahh yes after a careful re-read bonnieblktrk is right. Second option is to pick up some new bars. I pulled a set outta the 5-dollar bin at a little harley shop a while back that we're almost the same as the stock bonnie bars. Just make sure if you get new ones they are 1 inch and you're good.