There is no concept of inheritance among NIB files. Your only option is to duplicate the original file and make the necessary changes.
Generally speaking, I use XIBs to layout the custom cell, set the class of the cell in the XIB to my custom class and go from there with it's implementation file.
However it's not possible (to my knowledge) to have a base XIB with you inherit from.
I think the cleanest approach would be to have a separate XIB for each Cell you design.On the implementation side, where you define the class files, you can use inheritance there.