Change Step size (changes to RigidBody)

I am looking to hopefully manipulate the step size of individual rigid bodies in the simulation. I have never done anything like this before, but it seems to me that making a manipulation in b2World or b2Island is probably my best bet.

I have worked with box2d in flash before, and was curious about making some changes to the box2d source in unity. It seems that is not possible, so i figured perhaps i could ask for assitance here and see how far we can get.

Without access to these is there any public variables to do such a thing, or is it possible to extend rigid body to override any of the fuctions (which i dont think would even be virtual, and dont know if are even public). Is the rigidbody component public, and i can write my own with needed changes?

I guess I'm just coming to see if anyone has any advice, or if my best bet is to start writing my own priative componets to do the minimal collision detection.