How to import audioclip without conversion?

Every time when I drag audio files in the asset folder, Unity converts my files even if original format is same as target format (for example vorbis), the inspector shows that imported file size is different from original size. That means unnecessary transcode -- losing quality and waste of time. If I import as PCM the build size will be very large. It is possible to import the file "as is" and only decompress in runtime, like www.GetAudioClip?

Thank you very much.