If the professor is an IEEE member (I assume that the conference was an IEEE conference), then they signed an ethics statement and is subject to IEEE censure. IEEE has a way to denounce someone that protects the whistle-blower. If this was indeed an IEEE conference, but your professor is not a member, then IEEE might still get involved. People have been "black-listed" by IEEE and were not allowed to submit to any IEEE conference for a certain number of years. Since this action is public, it is quite embarrassing for the institution. At the same time, I trust that IEEE procedures are good, protecting both you and your professor.
Ideally, your institution has an ombudsman or similar person that you can go to to share your suspicions with. Unfortunately, often this ideal is not achieved and I will not assume that you (and your professor) would be protected by clear rules of proceeding and confidentiality.
Now, before you go to IEEE, you should do your homework. Write up your case, provide documentation (the conference should have access to your original submission, etc.) and why you are so certain that this is not an isolated incidence. You might also explain why the inserted citations are not appropriate. An obvious defense is that a professor is supposed to help students with publications, and this includes adding pertinent references. Once you submitted your request, you need to leave it to them. That can be very hard.