First, let's clarify the situation here. When you say "If the professor wants, he can give me a non passing grade [...]", do you mean that your institution has a specific policy that allows the professor to give you a non-passing grade at his discretion (based on a specific request from you, I assume) and without regard for your actual performance in the class? I would be very surprised if that were the case, but in the unlikely event that I'm wrong about this and there is such a policy, by all means go right ahead and ask (politely, while doing your best to pretend that the professor is human, as @Davidmh said in his funny comment).
In the more realistic case in which by "If the professor wants" you simply meant that the professor, being the person in charge, appears to you to have the authority to give you a failing grade, I would say that your interpretation of the situation is incorrect. Your request is unethical, and the professor actually has no right to give you any grade except the one that he/she thinks you deserve based on your performance in the class. Doing anything else would be dishonest and unethical. Think about it: what you are asking the professor to do is to
- breach the confidence of his employer, who pays him to assess your performance and output a grade that reflects his honest opinion of that performance, and
- to subvert the policies of the university, which, whether your professor agrees with them or not, were put in place by people who are in a decision-making authority and it is not his/her place to undermine, certainly not in such a surreptitious, dishonest way.
Given this, I am afraid that your request is highly inappropriate, and obviously there is no "good" way to make it. The policy that leads students to make such bizarre requests may or may not be a sensible one, and it would be appropriate if you wanted to ask the professor to help you fight it by, for example, writing a letter asking for an exception to the policy so that you can try to improve your grade in the class. But I don't see how you think it is even remotely reasonable to ask the professor to submit a dishonest grade. By the way, I had a student once make a similar request of me, and needless to say I refused, with the same reasoning as I explained above.