Prefab lost connection with origin resources when imported using AssetDatabase.importPackage

hi, i am writing a tool that will auto import unitypackage resources from develop project to productional project. AssetDatabase.import works fine on new resources. However, when overwrite existing files, prefabs would lose connection with some mesh or animation clip etc. Therefore, i need a api to extract the resource file list of the unitypackage in my script, which i did not find in the Unity Manual.

Any Advices? I am trying to use SharpZibLib to extract files from package, remove the existing files.