Yes it is possible, e.g. using PowerMock.mockStatic()
or PowerMock.mockStaticPartial()
if you want to mock one method, only.
There is a quite good tutorial on the homepage of PowerMock.
I posted an example on SO.
[EDIT] IF you want to do such magic in production code, however, I would recommend NOT to use a Testing-Framework. Maybe Groovy can help you with that - it is possible with Groovy to manipulate classes (even JDK classes) at runtime, and it is (I think) a thinner layer on top of the JVM.