How to ask for a non passing grade? In one course I got a lower than expected grade. If the professor wants, he can give me a non passing grade and this won't be on my transcript (non passing grades are not on our transcript). Of course I have to retake the course.

Many of our professors do it, but a few usually doesn't.

How I can polity ask him to give me a non passing grade?
I cannot say I agree with the practice: students should be responsible to know what they're doing as regards their studies, which, among other things, implies that they must have a sense as to how well-prepared they are for an exam (and they should also bear the risk and the consequences of occasionally making a wrong assessment). By setting a grade-threshold and ask the professor to give a non-passing grade if the threshold is not reached, the student effectively sheds this burden of responsibility off him or herself.  

That said, the usual practice is to ask such things before grading (for example if the exam is in paper and not electronic, students note that on the paper they hand in). Asking for it afterwards... well, if the professor is friendly to the practice, start by apologizing for not letting him know in advance and then ask what you want to ask.  

But if the professor is not friendly to the practice, consider living with the consequences of a lower-than-expected grade in your transcript.