Is it impolite to ask a professor who was not my master's thesis advisor (but with similar research field) to help me to work on a research topic?
Not at all. You are not committed to doing your PhD with the same advisor. Even if you did want to work with the same advisor, he/she should encourage that you talk to others to hear out their ideas. I highly doubt your advisor would be offended to hear that you were interested in speaking to other professors about their research interests. After all, a PhD is a long road, and you need to be really happy with your advisor, and your work, to survive it. 

In fact, I think it looks positive, even to your advisor, knowing that you have multiple options. I once had a job interview where, upon disclosing that I was also interviewing at other places, that "they would be worried if I wasn't".