You should be able to just assign the shader to the 'rim shader' input of the Crater Shader. I suppose that means you need to use the Crater Shader after your other shaders that you don't want on the rim.
If that doesn't work, you can use another copy of your Crater Shader to create a black and white mask. To do this, attach a white shader as the 'rim shader' (anything generating white will do, e.g a Default Shader or Lambert Shader). Then that crater with white at the rim can be used as a mask for your other shaders. To have more control over the mask, you could use a Colour Adjust Shader on the mask.
