Typically, the questions must be within the scope of the direct examination. However, any questions relating the credibility of the witness and his testimony are within the scope. If you want to question him on a completely different topic, you may have to call him as a witness in your defense case. But I would guess that anything you want to ask you would be able to do on cross.
In order to cross examine effectively takes years of practice and study. Chances are you are going to face a LEO with much more experience than you. Cops can be very evasive and difficult to control. They have practiced for years learning how to testify on cross examination without answering the question. Unless this is a simple speeding ticket/traffic case, hire an experienced criminal defense lawyer.