My PhD supervisor want to add his name on a paper from my MSc work I just started my PhD and I was planning to submit a paper from my MSc work to a conference soon. The paper was finished before I join my PhD program. 

Now my new PhD supervisor want to add his name and his post-doc name on the paper. He asked me to send the paper for his post-doc to review and then add both their names. Although my previous supervisor already reviewed the paper. 

How should I proceed? My new supervisor definitely has no contribution in the paper and feels like he just wants his name on more papers with no regard to how he contributed.
I'm in the social sciences, but I can't imagine that this is a discipline-specific issue. Is your new supervisor pre-tenure? If so, he may be pressured to have a stronger CV and publishing with his post-doc would look extra favorable. Regardless, (from your description) this appears to be an inappropriate request and I'm sorry that you were put in this predicament. I find the fact that the supervisor wants the post-doc to also co-author is especially troubling. 

First, I echo that you should find a new supervisor. If there is someone else whose work is close or closer to your interest, you can ask to transfer supervisors and claim this as an academic request and not a personal one.  

Second, is your MSc a co-author? If so, you can state that the prior supervisor is the co-author and you do not feel comfortable adding additional co-authors when the previous supervisor put in much of the work.

If not, you can still claim that you did the majority of the work and that if neither your supervisor nor his post-doc have any significant additional contributions to your paper, you do not feel comfortable adding them as co-authors. To further decline with professionalism, you could explain to the supervisor that you have other paper drafts or other projects that you would like to publish and/or present at conferences...if the two are interested in co-authoring you could collaborate on revising something else to publish together.

If your new supervisor is somewhat junior, he may not know that this was inappropriate (I'm giving him the benefit of the doubt), so I would try to be as professional about this as possible. It also might not surprise other faculty or administrators that this person asked you to do this (there are always faculty members here and there who are known to minimize student contribution and credit, though unfortunate).  Good luck!!!