This is not possible with the standard keyboard. The "Go" and "Next" buttons are "contextual" - they are supposed to represent actions depending on the current state.
If there are more text boxes to fill, the "Next" should be displayed, if not, then "Go" should move the user to the next screen or action.
You can make your own custom keyboard that overrides this behavior but I would advise against it as it would end up confusing the users since they are already used to this behavior. This falls under the design principle: If it looks the same, it should act the same (Under the Simplify My Life section)