Customizing a custom inspector!

Hi,

I've asked this question in Answers site but nobody could help. I'm giving the forum a try.
http://answers.unity3d.com/questions/795717/customizing-a-custom-inspector.html

Is there a proper way of customizing a custom inspector from an asset vendor? For example, NGUI customizes the default transform inspector (for adding some useful buttons) and PrefabEvolution customizes the default gameobject inspector (for its own way of implementing stuff).

I still want to be able to add...

Customizing a custom inspector!