Well, unfortunately, those voices are all produced by Apple so I have no ability to add to them. I agree that the Daniel voice is the best one so far, but it does give a cricket feel. Generally, voice additions show up with each iOS update so I would suggest just taking a look for new ones whenever your device is updated.
I am planning to allow the announcement sound byte to be recorded or imported and I have been approached by the stadium announcer for the Atlanta Braves, Casey Motter, as a potential source of recording those. He has done some recording of announcements for travel teams in the Atlanta area and would like to expand.