How do you link a new audio track to video in iMovie 11?

Hi,

I record my video with a seperate mic, edit the audio in garage band and add the track to imovie as an mp3 via itunes. I can detach the original audio and sync the new one to the video. However the track stays green wont edit in the same way as it does when its a purple track. I need it to act as one so I can make all the edits and transitions I do when its a normal purple track. The only way round I have found is to export it first and then re edit it. However that doesnt seem right as you loose audio quality each time and its very long winded.

Can anyone help me link the the new audio with the video?