Any idea how to import *.cwb file?

That's probably a CakeWalk file. I don't know of any Mac software that will convert that into a usable format.

You need to get a QuickTime compatible file to use it in GB (AIFF, WAV, Mp3, AAC...)

thats a Cakewalk Bundle file; it holds all of the data from the song. you'll need a version of Sonar or Pro Audio or the like to open it. once it's opened the individual tracks can be exported WAV files.

Yes, get a friend who uses Cakewalk or Sonar to help you.

First have him make a mixed-down audio file of the whole project and save it as an MP3 or as regular CD audio so you can listen to the whole project and see what it's supposed to sound like.

Then have him export any MIDI tracks as a Standard MIDI file, which you can drag into the GarageBand window and then assign software instruments to each track.

Have your friend take every audio track and export it as a .WAV file.

Then you can drag the individual WAV files into new blank Real Instrument tracks in GarageBand, one at a time, and figure out how they need to be lined up to produce the expected result.

Save the new GarageBand project.

From that point you can cut and paste or re-record sections of the audio files, and you can edit the MIDI tracks any way you like.