Your question isn't very clear about what you actually want to do; but I my advise: be really carefully about making you so dependent on your mocking frameworks.
The point is: that WhiteBox class is in a package named internal for a reason. Should you ever decide to move to PowerMockito v2 ... you will find that the WhiteBox class is gone! Where moving forward isn't that interesting; as PowerMockito doesn't work with any decently recent version of Mockito.
Thus, my serious advise: you are writing your own code. Then simply learn how to create testable code; for example by watching these videos.
I have made the experience that EasyMock or Mockito are completely sufficient to test code that is written to be testable. Using PowerMock without thinking about such aspects is most likely leading you to create less-than-optimal production code.